feat: a "Max" button on the Send screen (closes #198)

Max fills in a token's balance, cut down to the 18 decimal places the
confirmation screen accepts, or for ETH the exact balance minus the fee
reserve the confirmation screen's balance check gates on. An ETH fee estimate
that finishes after the Send screen was left, or its address, holding,
recipient or amount changed, fills nothing in. The confirmation screen works a
max ETH amount out again from its own fee estimate and signs it with that
estimate's fee fields, so a fee that rose before signing cannot push amount
plus fee above the balance. validateTransfer() still gates every send, the
check that ETH covers a token send's fee included. Where there is nothing to
fill in, a flash message says why.

Model: opus-5-5

README.md

@@ -1467,6 +1467,17 @@ view would leave a wallet one click from deletion.
     - Amount input with current balance display, which reads
       `Current balance: unknown (SYMBOL)` for a token whose scale is unknown, as
       ConfirmTx's balance line does (see Unknown token scale)
+    - "Max" button beside the amount input, always in place. It fills in a
+      token's balance, cut down to the 18 decimal places ConfirmTx accepts for a
+      token that has more, or for ETH the exact balance minus the network fee
+      reserve that ConfirmTx's balance check gates on, never the rounded balance
+      shown above it. The ETH fee is estimated for the recipient entered, so it
+      asks for a recipient first; an estimate that finishes after the screen was
+      left or the address, holding or recipient changed fills nothing in. Where
+      there is nothing to fill in, a flash message says why: the balance does
+      not cover the fee, the fee could not be estimated, or the token's balance
+      is unknown or zero. Typing in the amount makes it an ordinary amount;
+      changing what to send clears an amount Max filled in
     - "Review" button, disabled until the recipient validates
 - **Transitions**:
     - "Review" (valid inputs, ENS resolved) → **ConfirmTx**
@@ -1483,7 +1494,14 @@ view would leave a wallet one click from deletion.
     - Token contract: full address + etherscan link (ERC-20 only)
     - From: blockie + color dot + full address + etherscan link + wallet title
     - To: blockie + color dot + full address + etherscan link + ENS name
-    - Amount: value + symbol (USD in parentheses)
+    - Amount: value + symbol (USD in parentheses). An ETH amount Send's "Max"
+      filled in is worked out again from this screen's own fee estimate when it
+      arrives, as the balance minus the reserve, and the transaction is signed
+      with that estimate's fee fields, so a fee fetched again at signing cannot
+      exceed what the amount leaves behind. The address keeps whatever part of
+      the reserve the transaction does not use. If the balance no longer covers
+      the fee, the amount is left as it was and the amount-plus-fee error below
+      blocks the send
     - Your balance: value + symbol (USD in parentheses), or `unknown (SYMBOL)`
       for a token whose scale is unknown
     - Network fee: "Estimating..." then two lines, or "Unable to estimate",

TODO.md

@@ -45,6 +45,20 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-05: The Send screen has a "Max" button
+  ([#198](https://git.eeqj.de/sneak/AutistMask/issues/198)). Emptying an ETH
+  address took guessing an amount and being refused by the confirmation screen's
+  balance check. Max fills in a token's whole balance, cut to the 18 decimal
+  places the confirmation screen accepts, or for ETH the exact balance minus the
+  fee reserve that check gates on, never the four-decimal balance shown; a fee
+  estimate that finishes after the Send screen was left, or its address, holding
+  or recipient changed, fills nothing in. The confirmation screen works a max
+  ETH amount out again from its own fee estimate and signs it with that
+  estimate's fee fields: fetched again at signing, a fee that had risen since
+  would leave amount plus fee above the balance, and the node would refuse the
+  send. A token's maximum is still refused when ETH cannot pay the fee. Where
+  there is nothing to fill in, a flash message says why.
+
 - 2026-10-05: A token scale of zero decimals is tested
   ([#325](https://git.eeqj.de/sneak/AutistMask/issues/325)).
   `resolveTokenDecimals()` already used a scale of 0 from the bundled list or

docs/README.md

@@ -240,7 +240,9 @@ screen. Tokens can also be added from Settings, under "Tracked Tokens".
 2. Select what to send (ETH, or any ERC-20 token with a balance on this address
    that survives the spam filters).
 3. Enter the recipient address or ENS name (e.g. `vitalik.eth`).
-4. Enter the amount.
+4. Enter the amount, or click "Max" to fill it in: a token's balance, cut to 18
+   decimal places, or your ETH balance minus the amount reserved for the network
+   fee.
 5. Click "Review" to see the confirmation screen.
 
 The confirmation screen shows:

src/popup/index.html

@@ -551,12 +551,20 @@
                             class="text-xs text-muted"
                         ></span>
                     </div>
-                    <input
+                    <div class="flex gap-1">
-                        type="text"
+                        <input
-                        id="send-amount"
+                            type="text"
-                        class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
+                            id="send-amount"
-                        placeholder="0.0"
+                            class="border border-border p-1 flex-1 min-w-0 font-mono text-sm bg-bg text-fg"
-                    />
+                            placeholder="0.0"
+                        />
+                        <button
+                            id="btn-send-max"
+                            class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
+                        >
+                            Max
+                        </button>
+                    </div>
                 </div>
                 <button
                     id="btn-send-review"

src/popup/views/confirmTx.js

@@ -43,6 +43,7 @@ const {
     FEE_UNAVAILABLE,
     feeReserveWei,
     feeEstimateWei,
+    maxEthAmount,
     validateTransfer,
 } = require("../../shared/txValidation");
 const { log } = require("../../shared/log");
@@ -54,6 +55,10 @@ let pendingTx = null;
 // filled in by estimateGas() when the estimate resolves or fails.
 let feeStatus = FEE_PENDING;
 let feeWei = null;
+// The fee fields a max ETH send is signed with: those of the estimate its
+// amount was derived from. Null for any other send, which ethers prices from
+// the node at signing time.
+let maxSendFees = null;
 
 function restore() {
     const d = state.viewData;
@@ -82,10 +87,24 @@ function valueWithUsd(text, usdAmount) {
     return text;
 }
 
+// The Amount line, with its USD value. A max ETH send's line is drawn again
+// once its amount is re-derived from the fee estimate.
+function renderAmount(txInfo) {
+    const isErc20 = txInfo.token !== "ETH";
+    const rawSymbol = isErc20 ? txInfo.tokenSymbol || "?" : nativeCurrency();
+    const price = isErc20 ? getPrice(rawSymbol) : getPrice("ETH");
+    const amountUsd = price ? parseFloat(txInfo.amount) * price : null;
+    $("confirm-amount").textContent = valueWithUsd(
+        txInfo.amount + " " + displaySymbol(rawSymbol),
+        amountUsd,
+    );
+}
+
 function show(txInfo) {
     pendingTx = txInfo;
     feeStatus = FEE_PENDING;
     feeWei = null;
+    maxSendFees = null;
 
     const isErc20 = txInfo.token !== "ETH";
     // The raw symbol is the price-table key; the capped one is what the
@@ -132,18 +151,11 @@ function show(txInfo) {
     );
     $("confirm-to-ens").classList.add("hidden");
 
-    // Amount (with inline USD)
+    renderAmount(txInfo);
-    const ethPrice = getPrice("ETH");
-    const tokenPrice = getPrice(rawSymbol);
-    const amountNum = parseFloat(txInfo.amount);
-    const price = isErc20 ? tokenPrice : ethPrice;
-    const amountUsd = price ? amountNum * price : null;
-    $("confirm-amount").textContent = valueWithUsd(
-        txInfo.amount + " " + symbol,
-        amountUsd,
-    );
 
     // Balance (with inline USD)
+    const ethPrice = getPrice("ETH");
+    const tokenPrice = getPrice(rawSymbol);
     if (isErc20) {
         // null is a balance whose scale nothing knows, not a balance of zero
         // (https://git.eeqj.de/sneak/AutistMask/issues/349). The send is
@@ -380,8 +392,8 @@ async function estimateGas(txInfo) {
         }
 
         // What the node will require to be reserved, which is what the gate
-        // must be: the send pins no fee fields, so it is broadcast as a
+        // must be: the send is broadcast as a type-2 transaction priced at
-        // type-2 transaction priced at maxFeePerGas.
+        // maxFeePerGas, which only a max ETH send pins (see below).
         const gasCostWei = feeReserveWei(gasLimit, feeData);
         if (gasCostWei === null) {
             throw new Error("no usable gas price from the provider");
@@ -419,6 +431,30 @@ async function estimateGas(txInfo) {
         }
         feeStatus = FEE_KNOWN;
         feeWei = gasCostWei;
+        // A max ETH send is the balance minus this estimate's reserve, not the
+        // Send screen's, and is signed with this estimate's fee fields: fees
+        // fetched again at signing could exceed the reserve it leaves, and the
+        // node would refuse it for want of funds. Where the balance no longer
+        // covers the fee, the amount is left as it is and the balance check
+        // below says so.
+        if (txInfo.max && txInfo.token === "ETH") {
+            const amount = maxEthAmount(txInfo.balance, gasCostWei);
+            if (amount !== null) {
+                txInfo.amount = amount;
+                renderAmount(txInfo);
+                // Priced as feeReserveWei() priced the reserve: maxFeePerGas,
+                // or gasPrice on a network with no type-2 pricing.
+                if (feeData.maxFeePerGas != null) {
+                    maxSendFees = {
+                        gasLimit,
+                        maxFeePerGas: feeData.maxFeePerGas,
+                        maxPriorityFeePerGas: feeData.maxPriorityFeePerGas,
+                    };
+                } else {
+                    maxSendFees = { gasLimit, gasPrice: feeData.gasPrice };
+                }
+            }
+        }
         renderValidation(txInfo);
     } catch (e) {
         log.errorf("gas estimation failed:", e.shortMessage || e.message);
@@ -432,18 +468,19 @@ async function estimateGas(txInfo) {
 }
 
 // Populate the transaction this send describes, enforce the fee bound against
-// the fees that were actually filled in, then sign and broadcast it. The send
+// the fees that were actually filled in, then sign and broadcast it. Apart
-// pins no fee fields, so ethers fills maxFeePerGas and the gas limit from what
+// from a max ETH send, which passes its estimate's fee fields as `fees`, the
-// the configured RPC node answers, with nothing otherwise bounding what a
+// send pins no fee fields, so ethers fills maxFeePerGas and the gas limit from
+// what the configured RPC node answers, with nothing otherwise bounding what a
 // hostile node can set — the dApp path's ceilings never reached this one.
 // Populating before the check is what makes assertWithinCeilings() see the
 // same numbers that would be signed; it throws an ApprovalMismatchError when
 // the product gasLimit × maxFeePerGas is over the bound, which the caller
 // shows in the reserved error area rather than sending.
-async function populateVerifyAndSend(connectedSigner, tx) {
+async function populateVerifyAndSend(connectedSigner, tx, fees = null) {
     let request;
     if (tx.token === "ETH") {
-        request = { to: tx.to, value: parseEther(tx.amount) };
+        request = { to: tx.to, value: parseEther(tx.amount), ...fees };
     } else {
         const contract = new Contract(tx.token, ERC20_ABI, connectedSigner);
         // The contract's decimals() is read to be COMPARED with the scale the
@@ -548,7 +585,11 @@ function init(_ctx) {
             const provider = getProvider(state.rpcUrl, state.networkId);
             const connectedSigner = signer.connect(provider);
 
-            tx = await populateVerifyAndSend(connectedSigner, pendingTx);
+            tx = await populateVerifyAndSend(
+                connectedSigner,
+                pendingTx,
+                maxSendFees,
+            );
 
             // Best-effort: clear decrypted secret after use.
             // Note: JS strings are immutable; this nulls the reference but

src/popup/views/send.js

@@ -22,10 +22,25 @@ const {
     truncateAmountNeverZero,
     isBelowOneMillionth,
 } = require("../../shared/amountDisplay");
-const { getAddress } = require("ethers");
+const {
+    feeReserveWei,
+    maxEthAmount,
+    maxTokenAmount,
+} = require("../../shared/txValidation");
+const { log } = require("../../shared/log");
+const { getAddress, parseEther } = require("ethers");
 
 const ZERO_ADDRESS = "0x0000000000000000000000000000000000000000";
 
+// Whether the amount field holds what Max filled in. The confirmation screen
+// re-derives a max ETH amount from its own fee estimate; typing in the field
+// makes it an ordinary amount again.
+let amountIsMax = false;
+
+// Counts the times the Send screen has opened, so a Max fee estimate started
+// before it was last opened fills nothing in.
+let sendScreenOpenings = 0;
+
 /**
  * Validate a destination address string.
  * Returns { valid: true } or { valid: false, error: "..." }.
@@ -229,9 +244,102 @@ function updateSendBalance() {
     }
 }
 
+// Fill the amount field with the most the selected holding can send: a
+// token's whole balance (cut to 18 decimal places), or for ETH the exact
+// balance minus the fee reserve the confirmation screen checks against, never
+// the rounded balance the screen shows. Where there is nothing to fill in, a
+// flash message says why.
+async function fillMaxAmount() {
+    const addr = currentAddress();
+    if (!addr) return;
+    const token = state.selectedToken || $("send-token").value;
+
+    if (token !== "ETH") {
+        const bal = tokenBalanceAndDecimals(addr, token).tokenBalance;
+        if (bal == null) {
+            showFlash("This token's balance is unknown.");
+            return;
+        }
+        const amount = maxTokenAmount(bal);
+        if (!(parseFloat(amount) > 0)) {
+            showFlash("This token's balance is zero.");
+            return;
+        }
+        $("send-amount").value = amount;
+        amountIsMax = true;
+        return;
+    }
+
+    // The fee is estimated for this recipient, as the confirmation screen
+    // estimates it: sending to a contract can cost more gas.
+    const to = $("send-to").value.trim();
+    if (!validateToAddress(to).valid) {
+        showFlash("Please enter a recipient address first.");
+        return;
+    }
+    const typed = $("send-amount").value;
+    const opening = sendScreenOpenings;
+    let feeWei = null;
+    try {
+        const provider = getProvider(state.rpcUrl, state.networkId);
+        const [feeData, gasLimit] = await Promise.all([
+            provider.getFeeData(),
+            provider.estimateGas({
+                from: addr.address,
+                to,
+                value: parseEther(addr.balance || "0"),
+            }),
+        ]);
+        feeWei = feeReserveWei(gasLimit, feeData);
+    } catch (e) {
+        log.errorf(
+            "max amount fee estimate failed:",
+            e.shortMessage || e.message,
+        );
+    }
+    // While the estimate was in flight the user left the screen (and perhaps
+    // opened it again), typed an amount, or changed the address, the holding
+    // or the recipient: what they did wins.
+    if (
+        state.currentView !== "send" ||
+        sendScreenOpenings !== opening ||
+        currentAddress()?.address !== addr.address ||
+        (state.selectedToken || $("send-token").value) !== token ||
+        $("send-to").value.trim() !== to ||
+        $("send-amount").value !== typed
+    ) {
+        return;
+    }
+
+    if (feeWei === null) {
+        showFlash("The network fee could not be estimated.");
+        return;
+    }
+    const amount = maxEthAmount(addr.balance, feeWei);
+    if (amount === null) {
+        showFlash("Your balance does not cover the network fee.");
+        return;
+    }
+    $("send-amount").value = amount;
+    amountIsMax = true;
+}
+
 function init(_ctx) {
     ctx = _ctx;
-    $("send-token").addEventListener("change", updateSendBalance);
+    $("send-token").addEventListener("change", () => {
+        // A filled-in maximum is the maximum of the holding it was filled in
+        // for.
+        if (amountIsMax) {
+            $("send-amount").value = "";
+            amountIsMax = false;
+        }
+        updateSendBalance();
+    });
+
+    $("btn-send-max").addEventListener("click", fillMaxAmount);
+    $("send-amount").addEventListener("input", () => {
+        amountIsMax = false;
+    });
 
     // Initial state: disable review button until address is entered
     $("btn-send-review").disabled = true;
@@ -308,6 +416,7 @@ function init(_ctx) {
             tokenSymbol: tokenSymbol,
             tokenBalance: tokenBalance,
             tokenDecimals: tokenDecimals,
+            max: amountIsMax,
         });
     });
 
@@ -318,7 +427,10 @@ function init(_ctx) {
     });
 }
 
+// Called each time the Send screen opens, with its fields cleared.
 function resetSendValidation() {
+    sendScreenOpenings++;
+    amountIsMax = false;
     const errorEl = $("send-to-error");
     const btn = $("btn-send-review");
     if (errorEl) errorEl.textContent = "";

src/shared/txValidation.js

@@ -1,4 +1,4 @@
-// Balance arithmetic for the transaction confirmation screen.
+// Balance arithmetic for the Send and transaction confirmation screens.
 //
 // Pure: no DOM, no network, no state. Everything is exact integer math on
 // 18-decimal fixed point (wei for ETH), so it can be unit tested directly
@@ -10,7 +10,7 @@
 // the token balance arrive as human decimal strings, so comparing them at a
 // common scale is exact.
 
-const { parseUnits } = require("ethers");
+const { parseUnits, formatEther } = require("ethers");
 
 const SCALE_DECIMALS = 18;
 
@@ -87,6 +87,28 @@ function toFixedPoint(value) {
     }
 }
 
+// The most ETH a send can carry: the exact balance minus the fee reserve from
+// feeReserveWei(), as a decimal string, so validateTransfer() passes it with
+// exactly that reserve left behind. `ethBalance` is the exact decimal string
+// balances.js stores, never a rounded one. Null when the balance does not
+// leave anything to send once the fee is paid, or when either input is
+// unusable.
+function maxEthAmount(ethBalance, feeWei) {
+    const balanceWei = toFixedPoint(ethBalance);
+    if (balanceWei === null) return null;
+    if (typeof feeWei !== "bigint" || feeWei < 0n) return null;
+    const amountWei = balanceWei - feeWei;
+    if (amountWei <= 0n) return null;
+    return formatEther(amountWei);
+}
+
+// The most of a token a send can carry: its balance cut down, never rounded
+// up, to the 18 places (SCALE_DECIMALS) an amount may have. A token can
+// declare more than 18 decimals, and its balance is stored with all of them.
+function maxTokenAmount(tokenBalance) {
+    return tokenBalance.replace(/(\.\d{18})\d+$/, "$1");
+}
+
 // Validate a pending transfer against the balances that must cover it.
 //
 //   isErc20      — token transfer rather than a native ETH transfer
@@ -139,13 +161,12 @@ function validateTransfer({
     const feeFp = known ? feeWei : null;
 
     if (isErc20) {
-        // A token can declare more than 18 decimals, and its balance is
+        // Only the first 18 places of the balance are read: an amount with
-        // stored with all of them. Only the first 18 places (SCALE_DECIMALS)
+        // more was refused above, so the places after them cannot decide
-        // are read: an amount with more was refused above, so the places
+        // whether the amount fits.
-        // after them cannot decide whether the amount fits.
         const tokenText =
             typeof tokenBalance === "string"
-                ? tokenBalance.replace(/(\.\d{18})\d+$/, "$1")
+                ? maxTokenAmount(tokenBalance)
                 : tokenBalance;
         const tokenFp = toFixedPoint(tokenText) ?? 0n;
         if (amountFp > tokenFp) codes.push(CODES.INSUFFICIENT_TOKEN);
@@ -174,6 +195,8 @@ module.exports = {
     SCALE_DECIMALS,
     feeReserveWei,
     feeEstimateWei,
+    maxEthAmount,
+    maxTokenAmount,
     toFixedPoint,
     validateTransfer,
 };

tests/sendMax.test.js

@@ -0,0 +1,530 @@
+// The Send screen's "Max" control
+// (https://git.eeqj.de/sneak/AutistMask/issues/198).
+//
+// For ETH it fills in the exact balance minus the fee reserve the
+// confirmation screen's balance check gates on, never the four-decimal balance
+// the Send screen shows; the confirmation screen re-derives that amount from
+// its own estimate, and signs with that estimate's fee fields. For a token it
+// fills in the whole balance, and the check that ETH covers the fee still
+// applies.
+//
+// Driven through the real refreshBalances(), Send screen and confirmation
+// screen, Sign & Send included, with only the node, the explorer and the DOM
+// stubbed.
+
+"use strict";
+
+// What the stub node answers, and the signed transactions it was handed.
+const mockNode = {
+    balanceWei: 0n,
+    feeData: null,
+    broadcast: [],
+};
+
+// The token rows the stub explorer reports for the address.
+const mockExplorer = { items: [] };
+
+jest.mock("ethers", () => {
+    const actual = jest.requireActual("ethers");
+    class StubProvider {
+        async getBalance() {
+            return mockNode.balanceWei;
+        }
+        async lookupAddress() {
+            return null;
+        }
+        async getFeeData() {
+            return mockNode.feeData;
+        }
+        async estimateGas() {
+            return 21000n;
+        }
+        async getCode() {
+            return "0x";
+        }
+        async getTransactionCount() {
+            return 1;
+        }
+        async getNetwork() {
+            return { chainId: 1n };
+        }
+        async broadcastTransaction(signed) {
+            mockNode.broadcast.push(signed);
+            return { hash: "0x" + "ab".repeat(32) };
+        }
+    }
+    return {
+        ...actual,
+        JsonRpcProvider: StubProvider,
+        Network: { from: () => ({}) },
+    };
+});
+
+jest.mock("../src/shared/log", () => ({
+    log: {
+        debugf: () => {},
+        infof: () => {},
+        warnf: () => {},
+        errorf: () => {},
+    },
+    // The explorer's token list, which refreshBalances() also fetches.
+    debugFetch: jest.fn(async () => ({
+        ok: true,
+        status: 200,
+        json: async () => mockExplorer.items,
+    })),
+    urlOrigin: () => "",
+    setRuntimeDebug: () => {},
+    isDebug: () => false,
+}));
+
+// The wait screen polls for a receipt; these tests stop at the broadcast.
+jest.mock("../src/popup/views/txStatus", () => ({
+    showWait: jest.fn(),
+    showError: jest.fn(),
+}));
+
+// The confirmation screen's Etherscan label lookup is the only fetch() these
+// screens make; it fails, as it does offline.
+global.fetch = jest.fn(() => {
+    throw new Error("tests must not perform network requests");
+});
+
+const { makeStorageStub } = require("./support/storageStub");
+global.chrome = { storage: makeStorageStub(), runtime: { sendMessage() {} } };
+
+// A stub DOM: every id resolves to a recording element.
+const elements = new Map();
+
+function makeEl(id) {
+    const handlers = new Map();
+    return {
+        id,
+        textContent: "",
+        innerHTML: "",
+        value: "",
+        disabled: false,
+        style: {},
+        dataset: {},
+        classList: {
+            add() {},
+            remove() {},
+            toggle() {},
+            contains: () => false,
+        },
+        handlers,
+        children: [],
+        addEventListener(name, fn) {
+            handlers.set(name, fn);
+        },
+        appendChild(child) {
+            this.children.push(child);
+            return child;
+        },
+        querySelectorAll: () => [],
+        querySelector: () => null,
+        remove() {},
+        focus() {},
+    };
+}
+
+global.document = {
+    getElementById(id) {
+        if (!elements.has(id)) elements.set(id, makeEl(id));
+        return elements.get(id);
+    },
+    createElement: (tag) => makeEl(tag),
+    body: { prepend() {}, appendChild() {} },
+    addEventListener() {},
+};
+global.navigator = { clipboard: { writeText() {} } };
+
+const { Transaction, Wallet, formatEther } = require("ethers");
+const { refreshBalances } = require("../src/shared/balances");
+const { encryptWithPassword } = require("../src/shared/vault");
+const { state } = require("../src/shared/state");
+const send = require("../src/popup/views/send");
+const confirmTx = require("../src/popup/views/confirmTx");
+
+const PRIVATE_KEY = "0x" + "11".repeat(32);
+const HOLDER = new Wallet(PRIVATE_KEY).address;
+const RECIPIENT = "0xC0FfEE0000000000000000000000000000c0fFEe";
+// A second address of the wallet, and a second recipient.
+const OTHER = "0x" + "e".repeat(40);
+const PASSWORD = "correct horse battery staple";
+
+const GWEI = 1000000000n;
+const GAS = 21000n;
+
+// The Send screen shows this balance as 1.2345 ETH.
+const BALANCE_WEI = 1234567890123456789n;
+
+// A token the bundled list does not know, with enough holders to be listed.
+const TOKEN = "0x" + "d".repeat(40);
+
+// Fee data whose reserve is 21000 gas at `maxFeePerGas`. The expected cost,
+// at gasPrice, is lower, as it is on mainnet.
+function fees(maxFeePerGas) {
+    return {
+        maxFeePerGas,
+        maxPriorityFeePerGas: GWEI,
+        gasPrice: maxFeePerGas / 2n,
+    };
+}
+
+// The balance minus a reserve of 21000 gas at `maxFeePerGas`.
+function maxAfter(maxFeePerGas) {
+    return formatEther(BALANCE_WEI - GAS * maxFeePerGas);
+}
+
+function el(id) {
+    return global.document.getElementById(id);
+}
+
+function text(id) {
+    return el(id).textContent;
+}
+
+// The ETH balance the node reports and the token rows the explorer reports,
+// fetched and stored exactly where the popup stores them.
+async function refreshWith(balanceWei, tokenItems = []) {
+    mockNode.balanceWei = balanceWei;
+    mockExplorer.items = tokenItems;
+    state.wallets = [
+        {
+            type: "key",
+            name: "Wallet 1",
+            encryptedSecret: await encryptWithPassword(PRIVATE_KEY, PASSWORD),
+            addresses: [{ address: HOLDER }],
+        },
+    ];
+    state.selectedWallet = 0;
+    state.selectedAddress = 0;
+    await refreshBalances(
+        state.wallets,
+        "https://rpc.example.invalid",
+        "https://blockscout.example/api/v2",
+        state.trackedTokens,
+        "mainnet",
+    );
+}
+
+function tokenRow(value, decimals = "18") {
+    return {
+        value: String(value),
+        token: {
+            type: "ERC-20",
+            address_hash: TOKEN,
+            symbol: "TOK",
+            name: "Token",
+            decimals,
+            holders_count: "50000",
+        },
+    };
+}
+
+// The confirmation screen Review leads to, once shown.
+let confirmed = null;
+
+// Open the Send screen for `token` ("ETH" or a token address), with the
+// recipient entered.
+function openSend(token = "ETH") {
+    send.init({ showConfirmTx: (info) => (confirmed = info) });
+    confirmTx.init({});
+    send.resetSendValidation();
+    state.currentView = "send";
+    state.selectedToken = token;
+    el("send-to").value = RECIPIENT;
+    el("send-amount").value = "";
+}
+
+async function pressMax() {
+    await el("btn-send-max").handlers.get("click")();
+}
+
+// Press Review and show the confirmation screen with its fee estimate settled.
+async function review() {
+    await el("btn-send-review").handlers.get("click")();
+    confirmTx.show(confirmed);
+    await settle();
+}
+
+// show() starts the fee estimate without awaiting it; this lets it settle.
+async function settle() {
+    for (let i = 0; i < 10; i++) await new Promise((r) => setTimeout(r, 0));
+}
+
+function canSend() {
+    return !el("btn-confirm-send").disabled;
+}
+
+beforeEach(() => {
+    elements.clear();
+    confirmed = null;
+    state.selectedToken = null;
+    state.trackedTokens = [];
+    state.fraudContracts = [];
+    state.currentView = null;
+    mockNode.feeData = fees(20n * GWEI);
+    mockNode.broadcast = [];
+});
+
+describe("Max on an ETH send", () => {
+    test("fills in the exact balance minus the fee reserve", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        send.updateSendBalance();
+        expect(text("send-balance")).toBe("Current balance: 1.2345 ETH");
+        await pressMax();
+        // 1.234567890123456789 - 21000 * 20 gwei.
+        expect(el("send-amount").value).toBe("1.234147890123456789");
+    });
+
+    test("leaves exactly the fee reserve behind, and the confirmation screen enables Send", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        await pressMax();
+        await review();
+        expect(text("confirm-amount")).toBe(maxAfter(20n * GWEI) + " ETH");
+        expect(el("confirm-errors").innerHTML).toBe("");
+        expect(el("confirm-amount-fee-error").style.visibility).toBe("hidden");
+        expect(canSend()).toBe(true);
+    });
+
+    test("re-derives the amount when the fee estimate changes", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        await pressMax();
+        expect(el("send-amount").value).toBe(maxAfter(20n * GWEI));
+
+        // The fee rises between Max and the confirmation screen's estimate.
+        // Kept, the Send screen's amount would be refused for want of funds.
+        mockNode.feeData = fees(30n * GWEI);
+        await review();
+        expect(text("confirm-amount")).toBe(maxAfter(30n * GWEI) + " ETH");
+        expect(canSend()).toBe(true);
+
+        // And falls when the screen is shown again, as on reopening the popup.
+        mockNode.feeData = fees(10n * GWEI);
+        confirmTx.restore();
+        await settle();
+        expect(text("confirm-amount")).toBe(maxAfter(10n * GWEI) + " ETH");
+        expect(canSend()).toBe(true);
+    });
+
+    test("is signed with the fee its amount leaves behind", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        await pressMax();
+        await review();
+        // The fee the node quotes rises after the estimate. Fetched afresh
+        // at signing, it would make amount plus fee more than the balance.
+        mockNode.feeData = fees(25n * GWEI);
+        el("confirm-tx-password").value = PASSWORD;
+        await el("btn-confirm-send").handlers.get("click")();
+
+        expect(mockNode.broadcast).toHaveLength(1);
+        const tx = Transaction.from(mockNode.broadcast[0]);
+        expect(tx.to).toBe(RECIPIENT);
+        expect(tx.maxFeePerGas).toBe(20n * GWEI);
+        expect(tx.value + tx.gasLimit * tx.maxFeePerGas).toBe(BALANCE_WEI);
+    });
+
+    test("says so instead of filling in an amount when the balance does not cover the fee", async () => {
+        // 0.0001 ETH against a reserve of 0.00042 ETH.
+        await refreshWith(100000000000000n);
+        openSend();
+        await pressMax();
+        expect(el("send-amount").value).toBe("");
+        expect(text("flash-msg")).toBe(
+            "Your balance does not cover the network fee.",
+        );
+    });
+
+    test("asks for the recipient first, since the fee depends on it", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        el("send-to").value = "";
+        await pressMax();
+        expect(el("send-amount").value).toBe("");
+        expect(text("flash-msg")).toBe(
+            "Please enter a recipient address first.",
+        );
+    });
+
+    // Holds the node's fee answer, so Max's estimate is still running, until
+    // the returned function is called.
+    function holdFeeEstimate() {
+        let release;
+        mockNode.feeData = new Promise((resolve) => {
+            release = () => resolve(fees(20n * GWEI));
+        });
+        return release;
+    }
+
+    test.each([
+        ["the same address", 0],
+        ["another address", 1],
+    ])(
+        "fills nothing in once Send was left and opened again for %s while the fee was estimated",
+        async (_, addressIndex) => {
+            await refreshWith(BALANCE_WEI);
+            state.wallets[0].addresses.push({
+                address: OTHER,
+                balance: "2.0",
+                tokenBalances: [],
+            });
+            openSend();
+            const release = holdFeeEstimate();
+            const pressed = pressMax();
+
+            // Back, then Send again as home.js opens it, with the same
+            // recipient typed in again.
+            state.selectedAddress = addressIndex;
+            el("send-to").value = "";
+            el("send-amount").value = "";
+            send.resetSendValidation();
+            el("send-to").value = RECIPIENT;
+
+            release();
+            await pressed;
+            expect(el("send-amount").value).toBe("");
+            expect(text("flash-msg")).toBe("");
+        },
+    );
+
+    test("fills nothing in and says nothing once Send was left while the fee was estimated", async () => {
+        // 0.0001 ETH, which does not cover the fee: a result that landed
+        // would say so on whichever screen is shown.
+        await refreshWith(100000000000000n);
+        openSend();
+        const release = holdFeeEstimate();
+        const pressed = pressMax();
+        state.currentView = "home";
+        release();
+        await pressed;
+        expect(el("send-amount").value).toBe("");
+        expect(text("flash-msg")).toBe("");
+    });
+
+    test("fills nothing in when the recipient changed while the fee was estimated", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        const release = holdFeeEstimate();
+        const pressed = pressMax();
+        el("send-to").value = OTHER;
+        release();
+        await pressed;
+        expect(el("send-amount").value).toBe("");
+        expect(text("flash-msg")).toBe("");
+    });
+
+    test("fills nothing in when the holding was changed while the fee was estimated", async () => {
+        await refreshWith(BALANCE_WEI, [tokenRow(10n ** 18n)]);
+        // Opened from the home screen, where the dropdown picks the holding.
+        openSend(null);
+        el("send-token").value = "ETH";
+        const release = holdFeeEstimate();
+        const pressed = pressMax();
+        el("send-token").value = TOKEN;
+        el("send-token").handlers.get("change")();
+        release();
+        await pressed;
+        expect(el("send-amount").value).toBe("");
+        expect(text("flash-msg")).toBe("");
+    });
+
+    test("keeps an amount typed while the fee was estimated", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        const release = holdFeeEstimate();
+        const pressed = pressMax();
+        el("send-amount").value = "0.5";
+        el("send-amount").handlers.get("input")();
+        release();
+        await pressed;
+        expect(el("send-amount").value).toBe("0.5");
+        expect(text("flash-msg")).toBe("");
+    });
+
+    test("fills in once the held fee estimate arrives with nothing changed", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        const release = holdFeeEstimate();
+        const pressed = pressMax();
+        release();
+        await pressed;
+        expect(el("send-amount").value).toBe(maxAfter(20n * GWEI));
+    });
+
+    test("typed over, is an ordinary amount the confirmation screen keeps", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        await pressMax();
+        el("send-amount").value = "0.5";
+        el("send-amount").handlers.get("input")();
+        mockNode.feeData = fees(30n * GWEI);
+        await review();
+        expect(text("confirm-amount")).toBe("0.5 ETH");
+    });
+});
+
+describe("Max on a token send", () => {
+    // 1234.567890123456789012 TOK; the Send screen shows 1234.5678.
+    const TOKEN_UNITS = 1234567890123456789012n;
+
+    test("fills in the whole token balance", async () => {
+        await refreshWith(BALANCE_WEI, [tokenRow(TOKEN_UNITS)]);
+        openSend(TOKEN);
+        await pressMax();
+        expect(el("send-amount").value).toBe("1234.567890123456789012");
+        await review();
+        expect(text("confirm-amount")).toBe("1234.567890123456789012 TOK");
+        expect(canSend()).toBe(true);
+    });
+
+    test("of a token with more than 18 decimal places, fills in the balance cut down to the 18 the confirmation screen accepts", async () => {
+        // 1234.567890123456789012999999 TOK at 24 decimal places.
+        await refreshWith(BALANCE_WEI, [
+            tokenRow(1234567890123456789012999999n, "24"),
+        ]);
+        openSend(TOKEN);
+        await pressMax();
+        expect(el("send-amount").value).toBe("1234.567890123456789012");
+        await review();
+        expect(text("confirm-amount")).toBe("1234.567890123456789012 TOK");
+        expect(canSend()).toBe(true);
+    });
+
+    test("is still refused when ETH cannot cover the fee", async () => {
+        await refreshWith(0n, [tokenRow(TOKEN_UNITS)]);
+        openSend(TOKEN);
+        await pressMax();
+        expect(el("send-amount").value).toBe("1234.567890123456789012");
+        await review();
+        expect(el("confirm-gas-error").style.visibility).toBe("visible");
+        expect(canSend()).toBe(false);
+    });
+
+    test("says so when the token balance is unknown", async () => {
+        // No scale from the explorer, the bundled list or a tracked token.
+        const row = tokenRow(TOKEN_UNITS);
+        delete row.token.decimals;
+        await refreshWith(BALANCE_WEI, [row]);
+        openSend(TOKEN);
+        await pressMax();
+        expect(el("send-amount").value).toBe("");
+        expect(text("flash-msg")).toBe("This token's balance is unknown.");
+    });
+
+    test("says so when the token balance is zero", async () => {
+        state.trackedTokens = [
+            { address: TOKEN, symbol: "TOK", name: "Token", decimals: 18 },
+        ];
+        await refreshWith(BALANCE_WEI, [tokenRow(0n)]);
+        openSend(TOKEN);
+        await pressMax();
+        expect(el("send-amount").value).toBe("");
+        expect(text("flash-msg")).toBe("This token's balance is zero.");
+    });
+});

tests/txValidation.test.js

@@ -6,6 +6,8 @@ const {
     FEE_UNAVAILABLE,
     feeReserveWei,
     feeEstimateWei,
+    maxEthAmount,
+    maxTokenAmount,
     toFixedPoint,
     validateTransfer,
 } = require("../src/shared/txValidation");
@@ -306,6 +308,72 @@ describe("feeEstimateWei", () => {
     });
 });
 
+// The amount the Send screen's Max fills in for ETH: the exact balance, as
+// balances.js stores it, minus the fee reserve. Never the four-decimal balance
+// the Send screen shows.
+describe("maxEthAmount", () => {
+    // The Send screen shows this balance as 1.2345.
+    const BALANCE = "1.234567890123456789";
+
+    test("is the exact balance minus the fee, to the wei", () => {
+        expect(maxEthAmount(BALANCE, FEE)).toBe("1.234147890123456789");
+        expect(
+            parseEther(BALANCE) - parseEther(maxEthAmount(BALANCE, FEE)),
+        ).toBe(FEE);
+    });
+
+    test("passes validateTransfer with exactly the fee left behind", () => {
+        const r = validateTransfer({
+            isErc20: false,
+            amount: maxEthAmount(BALANCE, FEE),
+            ethBalance: BALANCE,
+            feeStatus: FEE_KNOWN,
+            feeWei: FEE,
+        });
+        expect(r).toEqual({ canSend: true, codes: [] });
+    });
+
+    test("is one wei when the balance is one wei more than the fee", () => {
+        expect(maxEthAmount("0.000420000000000001", FEE)).toBe(
+            "0.000000000000000001",
+        );
+    });
+
+    test("is null when the balance does not cover the fee", () => {
+        expect(maxEthAmount("0.0001", FEE)).toBe(null);
+        expect(maxEthAmount("0.0", FEE)).toBe(null);
+    });
+
+    test("is null when the balance covers the fee and nothing more", () => {
+        expect(maxEthAmount("0.00042", FEE)).toBe(null);
+    });
+
+    test("is null on a balance or fee it cannot do exact arithmetic on", () => {
+        expect(maxEthAmount(undefined, FEE)).toBe(null);
+        expect(maxEthAmount("not a number", FEE)).toBe(null);
+        expect(maxEthAmount(BALANCE, null)).toBe(null);
+        expect(maxEthAmount(BALANCE, -1n)).toBe(null);
+        expect(maxEthAmount(BALANCE, 420000000000000)).toBe(null);
+    });
+});
+
+// The amount the Send screen's Max fills in for a token.
+describe("maxTokenAmount", () => {
+    test("cuts a balance with more than 18 places down, never up", () => {
+        const amount = maxTokenAmount("1234.567890123456789012999999");
+        expect(amount).toBe("1234.567890123456789012");
+        expect(toFixedPoint(amount)).not.toBe(null);
+    });
+
+    test("leaves a balance with 18 places or fewer as it is", () => {
+        expect(maxTokenAmount("0.123456789012345678")).toBe(
+            "0.123456789012345678",
+        );
+        expect(maxTokenAmount("1.5")).toBe("1.5");
+        expect(maxTokenAmount("100")).toBe("100");
+    });
+});
+
 // Everything that is not a usable fee blocks exactly as FEE_UNAVAILABLE does.
 // Each of these previously returned { canSend: true, codes: [] } — counting no
 // fee at all, on a full-balance send, in the direction that lets money out.