feat: decide the AI edit's No change signal from the value the composer will actually hold rather than from the model's answer, since a rewrite whose every added character falls past the body-length cap leaves the textarea exactly as it was and reporting Rewritten over an unchanged body is the one thing that signal exists to prevent

web/src/components/app/ai/TextareaAIEdit.tsx

@@ -241,10 +241,15 @@ export default function TextareaAIEdit({
             // go back on before anything is written or compared: what lands in
             // the box IS what "did it change?" is answered from (issue #432).
             const applied = restoreEdges(target.text, text);
-            setChanged(applied !== target.text);
             const prefix = prevValue.slice(0, target.start);
             const suffix = prevValue.slice(target.end);
             const cap = (s: string) => (maxLen ? s.slice(0, maxLen) : s);
+            // Against the value that will actually be in the box, not against
+            // the model's answer: a rewrite that falls past the composer's cap
+            // leaves the body exactly as it was, and "Rewritten" over an
+            // unchanged body is the thing this signal exists to prevent.
+            const settled = cap(prefix + applied + suffix);
+            setChanged(settled !== prevValue);
             typewriter.run(
                 applied,
                 (partial) => {
@@ -263,7 +268,6 @@ export default function TextareaAIEdit({
                     // What the box holds, not what the model sent: maxLen can
                     // cut the tail off, and a range recorded past the end would
                     // have Undo and Again working on text that is not there.
-                    const settled = cap(prefix + applied + suffix);
                     const end = Math.min(target.start + applied.length, settled.length);
                     const newRange = { start: target.start, end, text: settled.slice(target.start, end) };
                     lastRun.current = { instruction, prevValue, start: target.start, origEnd: target.end };

web/src/components/app/ai/textareaAIEdit.test.tsx

@@ -44,13 +44,13 @@ import TextareaAIEdit from "./TextareaAIEdit";
 
 const MAX = 20;
 
-function Host({ initial }: { initial: string }) {
+function Host({ initial, max = MAX }: { initial: string; max?: number }) {
     const ref = React.useRef<HTMLTextAreaElement>(null);
     const [value, setValue] = React.useState(initial);
     return (
         <>
             <textarea ref={ref} value={value} onChange={(e) => setValue(e.target.value)} />
-            <TextareaAIEdit textareaRef={ref} value={value} onChange={setValue} maxLen={MAX} />
+            <TextareaAIEdit textareaRef={ref} value={value} onChange={setValue} maxLen={max} />
         </>
     );
 }
@@ -102,6 +102,33 @@ describe("TextareaAIEdit against the composer's length cap", () => {
         expect(sent?.text).toBe("two");
     });
 
+    it("says No change when the cap swallows the rewrite whole", async () => {
+        const { container } = render(
+            <QueryClientProvider client={new QueryClient()}>
+                <Host initial="abcdefg" max={7} />
+            </QueryClientProvider>,
+        );
+        const ta = container.querySelector("textarea")!;
+        await act(async () => {
+            ta.focus();
+            ta.setSelectionRange(5, 7);
+            document.dispatchEvent(new Event("selectionchange"));
+        });
+        // Everything this adds lands past the cap, so the body cannot move.
+        reply = { text: "fg extra", credits_charged: 1, tokens_used: 10 };
+        await act(async () => {
+            fireEvent.mouseDown(await screen.findByText("Edit with AI"));
+        });
+        const input = await screen.findByPlaceholderText("Tell AI how to change it…");
+        await act(async () => {
+            fireEvent.change(input, { target: { value: "expand" } });
+            fireEvent.keyDown(input, { key: "Enter" });
+        });
+        expect(ta.value).toBe("abcdefg");
+        await screen.findByText("No change");
+        expect(screen.queryByText("Rewritten")).toBeNull();
+    });
+
     it("leaves Undo pointing at the words that were selected, not a derived range", async () => {
         const initial = "one two three";
         const { container } = render(