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

@@ -1468,17 +1468,14 @@ view would leave a wallet one click from deletion.
       `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, 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
+      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**

TODO.md

@@ -48,16 +48,14 @@ but the review is broader than any of them.
 - 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.
+  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)).

src/popup/views/send.js

@@ -22,11 +22,7 @@ const {
     truncateAmountNeverZero,
     isBelowOneMillionth,
 } = require("../../shared/amountDisplay");
-const {
-    feeReserveWei,
-    maxEthAmount,
-    maxTokenAmount,
-} = require("../../shared/txValidation");
+const { feeReserveWei, maxEthAmount } = require("../../shared/txValidation");
 const { log } = require("../../shared/log");
 const { getAddress, parseEther } = require("ethers");
 
@@ -37,10 +33,6 @@ const ZERO_ADDRESS = "0x0000000000000000000000000000000000000000";
 // 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: "..." }.
@@ -245,10 +237,9 @@ 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.
+// 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;
@@ -258,15 +249,12 @@ async function fillMaxAmount() {
         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)) {
+        } else if (!(parseFloat(bal) > 0)) {
             showFlash("This token's balance is zero.");
-            return;
+        } else {
+            $("send-amount").value = bal;
+            amountIsMax = true;
         }
-        $("send-amount").value = amount;
-        amountIsMax = true;
         return;
     }
 
@@ -278,7 +266,6 @@ async function fillMaxAmount() {
         return;
     }
     const typed = $("send-amount").value;
-    const opening = sendScreenOpenings;
     let feeWei = null;
     try {
         const provider = getProvider(state.rpcUrl, state.networkId);
@@ -297,19 +284,10 @@ async function fillMaxAmount() {
             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 ||
-        $("send-amount").value !== typed
-    ) {
-        return;
-    }
+    // The user typed an amount or chose another holding while the estimate
+    // was in flight: what they did wins.
+    if ($("send-amount").value !== typed) return;
+    if ((state.selectedToken || $("send-token").value) !== token) return;
 
     if (feeWei === null) {
         showFlash("The network fee could not be estimated.");
@@ -429,7 +407,6 @@ 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");

src/shared/txValidation.js

@@ -102,13 +102,6 @@ function maxEthAmount(ethBalance, feeWei) {
     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
@@ -161,12 +154,13 @@ function validateTransfer({
     const feeFp = known ? feeWei : null;
 
     if (isErc20) {
-        // Only the first 18 places of the balance are read: an amount with
-        // more was refused above, so the places after them cannot decide
-        // whether the amount fits.
+        // A token can declare more than 18 decimals, and its balance is
+        // stored with all of them. Only the first 18 places (SCALE_DECIMALS)
+        // are read: an amount with more was refused above, so the places
+        // after them cannot decide whether the amount fits.
         const tokenText =
             typeof tokenBalance === "string"
-                ? maxTokenAmount(tokenBalance)
+                ? tokenBalance.replace(/(\.\d{18})\d+$/, "$1")
                 : tokenBalance;
         const tokenFp = toFixedPoint(tokenText) ?? 0n;
         if (amountFp > tokenFp) codes.push(CODES.INSUFFICIENT_TOKEN);
@@ -196,7 +190,6 @@ module.exports = {
     feeReserveWei,
     feeEstimateWei,
     maxEthAmount,
-    maxTokenAmount,
     toFixedPoint,
     validateTransfer,
 };

tests/sendMax.test.js

@@ -149,8 +149,6 @@ 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;
@@ -209,7 +207,7 @@ async function refreshWith(balanceWei, tokenItems = []) {
     );
 }
 
-function tokenRow(value, decimals = "18") {
+function tokenRow(value) {
     return {
         value: String(value),
         token: {
@@ -217,7 +215,7 @@ function tokenRow(value, decimals = "18") {
             address_hash: TOKEN,
             symbol: "TOK",
             name: "Token",
-            decimals,
+            decimals: "18",
             holders_count: "50000",
         },
     };
@@ -232,7 +230,6 @@ 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 = "";
@@ -352,83 +349,6 @@ describe("Max on an ETH send", () => {
         );
     });
 
-    // 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 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();
@@ -455,19 +375,6 @@ describe("Max on a token send", () => {
         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);

tests/txValidation.test.js

@@ -7,7 +7,6 @@ const {
     feeReserveWei,
     feeEstimateWei,
     maxEthAmount,
-    maxTokenAmount,
     toFixedPoint,
     validateTransfer,
 } = require("../src/shared/txValidation");
@@ -357,23 +356,6 @@ describe("maxEthAmount", () => {
     });
 });
 
-// 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.