fix(transcript): refuse every native edit the block does not handle

src/components/ai-edition/RightPanes.tsx

@@ -1309,8 +1309,14 @@ const TranscriptClipBlock = memo(function TranscriptClipBlock({
 			// their own typing bubbles here natively — React's `stopPropagation` only ever
 			// stopped the synthetic tree. Their text is theirs.
 			if (event.target instanceof HTMLInputElement) return;
+			// Default-deny: the block is contentEditable only to carry a caret, so no native
+			// edit may land in it. Not only the two families handled below: Ctrl/Cmd+B, I and U
+			// reach here as `formatBold`, `formatItalic` and `formatUnderline`, and let through
+			// they styled the DOM behind `words`' back (#1038). Undo loses nothing: Ctrl/Cmd+Z
+			// goes to the document undo (keydown, or the Edit menu on macOS), never to the
+			// browser's text history, which has nothing of this block's edits in it anyway.
+			event.preventDefault();
 			if (event.inputType.startsWith("delete")) {
-				event.preventDefault();
 				cutNativeSelection(event.inputType === "deleteContentForward" ? "forward" : "backward");
 				return;
 			}
@@ -1319,10 +1325,7 @@ const TranscriptClipBlock = memo(function TranscriptClipBlock({
 			// instead is a field beside the word the caret was on, whose commit creates a real
 			// word to hold them. So the gesture is the document one — put the caret somewhere
 			// and type — without the DOM ever getting ahead of `words`.
-			if (event.inputType.startsWith("insert")) {
-				event.preventDefault();
-				openInsertion(event.data ?? "");
-			}
+			if (event.inputType.startsWith("insert")) openInsertion(event.data ?? "");
 		};
 		editor.addEventListener("beforeinput", onBeforeInput);
 		return () => editor.removeEventListener("beforeinput", onBeforeInput);

src/components/ai-edition/TranscriptPane.wordInsert.test.tsx

@@ -291,3 +291,43 @@ describe("a word that was inserted", () => {
 		expect(view.onAddTrimRange).not.toHaveBeenCalled();
 	});
 });
+
+describe("the native edits the block does not handle", () => {
+	/** Dispatch a native `beforeinput`; true when the listener let the browser go ahead. */
+	function input(editor: HTMLElement, inputType: string, data?: string) {
+		return fireEvent(
+			editor,
+			new InputEvent("beforeinput", { inputType, data, bubbles: true, cancelable: true }),
+		);
+	}
+
+	// Ctrl/Cmd+B, I and U: let through, they styled the words in the DOM and `words` never
+	// knew (#1038). Undo and redo belong to the document, never to the browser's text history.
+	it.each([
+		"formatBold",
+		"formatItalic",
+		"formatUnderline",
+		"historyUndo",
+		"historyRedo",
+	])("refuses %s", (inputType) => {
+		const view = renderPane();
+		caretBeforeWordAt(view.editor, 2);
+		expect(input(view.editor, inputType)).toBe(false);
+		expect(view.field()).toBeNull();
+		expect(view.onAddTrimRange).not.toHaveBeenCalled();
+	});
+
+	it("still cuts on a delete", () => {
+		const view = renderPane();
+		caretBeforeWordAt(view.editor, 2);
+		expect(input(view.editor, "deleteContentBackward")).toBe(false);
+		expect(view.onAddTrimRange).toHaveBeenCalledTimes(1);
+	});
+
+	it("still opens the field on an insert", () => {
+		const view = renderPane();
+		caretBeforeWordAt(view.editor, 2);
+		expect(input(view.editor, "insertText", "v")).toBe(false);
+		expect(view.field()).toHaveValue("v");
+	});
+});