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

Max fills in a token's whole balance, or for ETH the exact balance minus
the fee reserve the confirmation screen's balance check gates on. 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 +1467,15 @@ 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 the
+      most the selected holding can send: a token's whole balance, 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.
+      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 +1492,14 @@ 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 +45,18 @@ 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, or for ETH the exact
+  balance minus the fee reserve that check gates on, never the four-decimal
+  balance shown. 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 +240,8 @@ 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 in everything you can send: a
+   token's whole balance, or your ETH balance minus 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
-                        type="text"
-                        id="send-amount"
-                        class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
-                        placeholder="0.0"
-                    />
+                    <div class="flex gap-1">
+                        <input
+                            type="text"
+                            id="send-amount"
+                            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)
-    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,
-    );
+    renderAmount(txInfo);
 
     // 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
-        // type-2 transaction priced at maxFeePerGas.
+        // must be: the send is broadcast as a type-2 transaction priced at
+        // 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
-// 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
+// the fees that were actually filled in, then sign and broadcast it. Apart
+// from a max ETH send, which passes its estimate's fee fields as `fees`, the
+// 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 +22,17 @@ const {
     truncateAmountNeverZero,
     isBelowOneMillionth,
 } = require("../../shared/amountDisplay");
-const { getAddress } = require("ethers");
+const { feeReserveWei, maxEthAmount } = 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;
+
 /**
  * Validate a destination address string.
  * Returns { valid: true } or { valid: false, error: "..." }.
@@ -229,9 +236,88 @@ function updateSendBalance() {
     }
 }
 
+// Fill the amount field with the most the selected holding can send: a
+// token's whole balance, 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.");
+        } else if (!(parseFloat(bal) > 0)) {
+            showFlash("This token's balance is zero.");
+        } else {
+            $("send-amount").value = bal;
+            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 = $("send-amount").value;
+    let feeWei = null;
+    try {
+        const provider = getProvider(state.rpcUrl, state.networkId);
+        const [feeData, gasLimit] = await Promise.all([
+            provider.getFeeData(),
+            provider.estimateGas({
+                from: addr.address,
+                to,
+                value: parseEther(addr.balance || "0"),
+            }),
+        ]);
+        feeWei = feeReserveWei(gasLimit, feeData);
+    } catch (e) {
+        log.errorf(
+            "max amount fee estimate failed:",
+            e.shortMessage || e.message,
+        );
+    }
+    // The user typed an amount or chose another holding while the estimate
+    // was in flight: what they did wins.
+    if ($("send-amount").value !== typed) return;
+    if ((state.selectedToken || $("send-token").value) !== token) 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 +394,7 @@ function init(_ctx) {
             tokenSymbol: tokenSymbol,
             tokenBalance: tokenBalance,
             tokenDecimals: tokenDecimals,
+            max: amountIsMax,
         });
     });
 
@@ -318,7 +405,9 @@ function init(_ctx) {
     });
 }
 
+// Called each time the Send screen opens, with its fields cleared.
 function resetSendValidation() {
+    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 +87,21 @@ 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);
+}
+
 // Validate a pending transfer against the balances that must cover it.
 //
 //   isErc20      — token transfer rather than a native ETH transfer
@@ -174,6 +189,7 @@ module.exports = {
     SCALE_DECIMALS,
     feeReserveWei,
     feeEstimateWei,
+    maxEthAmount,
     toFixedPoint,
     validateTransfer,
 };

tests/sendMax.test.js

@@ -0,0 +1,409 @@
+// 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";
+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) {
+    return {
+        value: String(value),
+        token: {
+            type: "ERC-20",
+            address_hash: TOKEN,
+            symbol: "TOK",
+            name: "Token",
+            decimals: "18",
+            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.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.",
+        );
+    });
+
+    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("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 +6,7 @@ const {
     FEE_UNAVAILABLE,
     feeReserveWei,
     feeEstimateWei,
+    maxEthAmount,
     toFixedPoint,
     validateTransfer,
 } = require("../src/shared/txValidation");
@@ -306,6 +307,55 @@ 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);
+    });
+});
+
 // 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.