JoshHeaps.Net/wwwroot/css/casino/holdem.css

@@ -3006,3 +3006,10 @@ textarea:focus-visible {
     color: #ecffef;
     font-weight: 600;
 }
+
+/* The quieter half of the deal prompt: things the deal is about to do that a dealer
+   would otherwise only discover afterwards. */
+.dealPromptLine.dealPromptAside {
+    font-size: 0.72rem;
+    color: #9a9a9a;
+}

JoshHeaps.Net/wwwroot/js/casino/holdem/actionBar.js

@@ -494,6 +494,17 @@ const ActionBar = {
             `${preview.headsUp ? " · heads-up" : ""}${preview.manual ? " · blinds set by hand" : ""}`;
         wrap.appendChild(line);
 
+        // Dealing relocates a button sitting on an empty chair. That has to be said
+        // before it happens rather than discovered afterwards — a fresh table starts
+        // the button on seat 1 whether or not anyone is sitting there.
+        if (preview.buttonMoves) {
+            const moved = document.createElement("p");
+            moved.className = "dealPromptLine dealPromptAside";
+            moved.textContent =
+                `Button moves to ${name(preview.buttonIndex)} — seat ${this.state.dealerSeatIndex + 1} is empty.`;
+            wrap.appendChild(moved);
+        }
+
         const row = document.createElement("div");
         row.className = "actionButtons";
         row.appendChild(this.button("Post blinds & deal", "actionDeal", () => {

JoshHeaps.Net/wwwroot/js/casino/holdem/betting.js

@@ -33,10 +33,22 @@ const Betting = {
     // while the table is still being set, so a dealer can see where they'll land
     // before any money moves.
     blindPreview(state) {
-        const { sbIndex, bbIndex, headsUp, manual } = HoldemState.getBlindSeats(state);
+        // Dealing relocates a button parked on an empty chair before it works out who
+        // posts, so the preview has to answer for the button where it *will* be. Asking
+        // from where it currently sits gave a different pair of blinds than the deal
+        // produced — the preview promising one thing and doing another is exactly the
+        // guessing this whole step exists to remove.
+        const buttonIndex = HoldemState.buttonSeatForDeal(state);
+        const asDealt = buttonIndex === state.dealerSeatIndex
+            ? state
+            : { ...state, dealerSeatIndex: buttonIndex };
+
+        const { sbIndex, bbIndex, headsUp, manual } = HoldemState.getBlindSeats(asDealt);
         if (sbIndex === null || bbIndex === null) return null;
         return {
             sbIndex, bbIndex, headsUp, manual: !!manual,
+            buttonIndex,
+            buttonMoves: buttonIndex !== state.dealerSeatIndex,
             smallBlind: this.round(state.smallBlind || 0),
             bigBlind: this.round(state.bigBlind || 0),
         };

JoshHeaps.Net/wwwroot/js/casino/holdem/main.js

@@ -96,13 +96,21 @@ document.addEventListener("DOMContentLoaded", () => {
             });
         },
 
-        makeDealer: index => seatAction(index, `Button on ${nameOf(index)}.`, () => {
-            state.dealerSeatIndex = index;
-            // Before anyone has acted the blinds are still just sitting there, so
-            // moving the button moves them too — a home game passes it round several
-            // times before the cards come out, and the posted chips have to follow.
-            if (Betting.onlyBlindsIn(state)) Betting.repostBlinds(state);
-        }),
+        makeDealer: index => {
+            // Before the deal there's nothing posted, so this is just moving a marker.
+            // After it, the blinds have to follow the button or the badges and the
+            // money disagree — but that's a real thing happening to real chips, and
+            // the dealer gets told rather than finding out later.
+            const willRepost = state.bettingOpen && Betting.onlyBlindsIn(state) &&
+                HoldemState.unresolvedContributions(state) > Betting.EPSILON_CHIPS;
+
+            seatAction(index, willRepost
+                ? `Button on ${nameOf(index)} — blinds taken back and re-posted.`
+                : `Button on ${nameOf(index)}.`, () => {
+                state.dealerSeatIndex = index;
+                if (willRepost) Betting.repostBlinds(state);
+            });
+        },
 
         assignBlind: (index, which) => {
             const blinds = HoldemState.getBlindSeats(state);

JoshHeaps.Net/wwwroot/js/casino/holdem/state.js

@@ -273,15 +273,25 @@ const HoldemState = {
     // position off it. On the very first hand of a fresh table it starts on seat 1
     // whether or not anybody is sitting there, and a table that empties out around it
     // leaves it stranded. Move it to a real player before a hand is dealt.
-    ensureButtonOnPlayer(state) {
-        if (state.seats[state.dealerSeatIndex] && state.seats[state.dealerSeatIndex].inHand) return;
+    // Where the button will be when the hand is dealt — without moving it.
+    //
+    // Kept separate from ensureButtonOnPlayer because the deal preview has to be able
+    // to ask the question without answering it. It couldn't, and so the preview lied:
+    // with the button parked on an empty chair it promised one pair of blinds and
+    // dealing produced another, because beginHand relocates the button first and then
+    // works out who posts.
+    buttonSeatForDeal(state) {
+        const seated = state.seats[state.dealerSeatIndex];
+        if (seated && seated.inHand) return state.dealerSeatIndex;
         for (let step = 0; step < state.seatCount; step++) {
             const index = this.nextSeatIndex(state, state.dealerSeatIndex, step);
-            if (state.seats[index].inHand) {
-                state.dealerSeatIndex = index;
-                return;
-            }
+            if (state.seats[index].inHand) return index;
         }
+        return state.dealerSeatIndex;
+    },
+
+    ensureButtonOnPlayer(state) {
+        state.dealerSeatIndex = this.buttonSeatForDeal(state);
     },
 
     // A hand-picked blind is only usable if that seat is actually in the hand — a

tests/holdem/dealStep.test.js

@@ -161,3 +161,45 @@ describe("a table saved mid-hand comes back mid-hand", () => {
         assert.equal(state.bettingOpen, false);
     });
 });
+
+describe("the preview tells the truth about the button", () => {
+    test("a button parked on an empty chair is answered for where it will be", () => {
+        // The preview used to read the button where it sat, while dealing relocated it
+        // first — so it promised one pair of blinds and produced another.
+        const state = makeTable({ seatCount: 6, seated: [1, 2, 3], dealerSeatIndex: 0 });
+        state.bettingOpen = false;
+        assert.equal(state.seats[0].inHand, false, "the button is on an empty chair");
+
+        const preview = Betting.blindPreview(state);
+        assert.equal(preview.buttonMoves, true, "and the preview knows it will move");
+
+        Betting.beginHand(state);
+
+        assert.equal(Betting.committed(state.seats[preview.sbIndex]), preview.smallBlind);
+        assert.equal(Betting.committed(state.seats[preview.bbIndex]), preview.bigBlind);
+        assert.equal(state.dealerSeatIndex, preview.buttonIndex, "and it moved where it said");
+    });
+
+    test("asking where the button will be doesn't move it", () => {
+        const state = makeTable({ seatCount: 6, seated: [1, 2, 3], dealerSeatIndex: 0 });
+        state.bettingOpen = false;
+
+        Betting.blindPreview(state);
+        Betting.blindPreview(state);
+
+        assert.equal(state.dealerSeatIndex, 0, "the preview is a question, not an action");
+    });
+
+    test("a button already on a player is left alone", () => {
+        const state = makeTable({ seatCount: 6, seated: [0, 1, 2], dealerSeatIndex: 0 });
+        state.bettingOpen = false;
+        const preview = Betting.blindPreview(state);
+        assert.equal(preview.buttonMoves, false);
+        assert.equal(preview.buttonIndex, 0);
+    });
+
+    test("buttonSeatForDeal copes with a table nobody is sitting at", () => {
+        const state = makeTable({ seatCount: 6, seated: [] });
+        assert.equal(HoldemState.buttonSeatForDeal(state), state.dealerSeatIndex);
+    });
+});