composer: unwrap quote blocks with Backspace

webviews/src/agent-session/acpmux/MarkdownField.tsx

@@ -22,7 +22,7 @@ import { commonmark } from "@milkdown/kit/preset/commonmark";
 import { gfm } from "@milkdown/kit/preset/gfm";
 import { history } from "@milkdown/kit/plugin/history";
 import { clipboard } from "@milkdown/kit/plugin/clipboard";
-import { baseKeymap, chainCommands, selectAll } from "@milkdown/kit/prose/commands";
+import { baseKeymap, chainCommands, lift, selectAll } from "@milkdown/kit/prose/commands";
 import { redo, redoDepth, undo, undoDepth } from "@milkdown/kit/prose/history";
 import { keymap } from "@milkdown/kit/prose/keymap";
 import { splitListItem } from "@milkdown/kit/prose/schema-list";
@@ -708,6 +708,23 @@ const MarkdownFieldImpl = React.forwardRef<MarkdownFieldHandle, MarkdownFieldPro
     });
     const blockKeys = $prose(() =>
       keymap({
+        // Milkdown replaces the base Backspace's joinBackward with joinTextblockBackward,
+        // which cannot lift a quote at the document's start and joins one into prior text.
+        // Lift its first paragraph instead, preserving text and the paragraph boundary.
+        Backspace: (state, dispatch) => {
+          const { selection } = state;
+          if (!(selection instanceof TextSelection) || !selection.empty) return false;
+          const { $from } = selection;
+          if (
+            $from.depth < 2 ||
+            $from.parentOffset !== 0 ||
+            $from.parent.type !== state.schema.nodes.paragraph ||
+            $from.node(-1).type !== state.schema.nodes.blockquote ||
+            $from.index(-1) !== 0
+          )
+            return false;
+          return lift(state, dispatch);
+        },
         "Shift-Enter": (state, dispatch, editorView) => {
           if (!lines.current) return true;
           const item = state.schema.nodes.list_item;

webviews/src/agent-session/acpmux/composerQuote.gallery.ts

@@ -0,0 +1,30 @@
+// l10n-allow-file: gallery fixtures, not shipped UI.
+// Leo 2026-10-26: "and i cant delete quote block text thingy with backspace? only shift enter? strange..."
+import { agentPaneEntry } from "../../gallery/format";
+import { chat } from "../../gallery/fixtures/acpmux";
+
+export default agentPaneEntry({
+  id: "agent-pane.composer-quote",
+  title: "Backspace out of a composer quote",
+  area: "Agent pane",
+  height: 420,
+  widths: { narrow: 400, normal: 760, wide: 1040 },
+  covers: ["agent-session/acpmux/MarkdownField.tsx"],
+  variants: {
+    text: {
+      note: "Place the caret at the quote's start and press Backspace: its text becomes a plain paragraph.",
+      ready: { draft: "> Keep this quote text" },
+      snapshot: chat([]),
+    },
+    empty: {
+      note: "Backspace removes an empty quote, leaving the composer ready for a plain paragraph.",
+      ready: { draft: ">" },
+      snapshot: chat([]),
+    },
+    "after-paragraph": {
+      note: "Unwrapping a quote keeps the earlier paragraph separate and preserves both texts.",
+      ready: { draft: "Earlier paragraph\n\n> Keep this quote text" },
+      snapshot: chat([]),
+    },
+  },
+});

webviews/test/composer-quote-backspace.test.tsx

@@ -0,0 +1,135 @@
+// Leo 2026-10-26: "and i cant delete quote block text thingy with backspace? only shift enter? strange..."
+// Drive the shared composer editor's real keymap: Backspace lifts quotes without deleting text.
+import { afterAll, afterEach, beforeAll, expect, test } from "bun:test";
+import { JSDOM } from "jsdom";
+import { act, createRef } from "react";
+import type { Root } from "react-dom/client";
+import type { MarkdownFieldHandle } from "../src/agent-session/acpmux/MarkdownField";
+import { promptField, proseMirrorGlobals } from "../src/agent-session/acpmux/promptFieldTesting";
+
+const scope = globalThis as Record<string, unknown>;
+const keys = [
+  "window",
+  "document",
+  "navigator",
+  "HTMLElement",
+  "IS_REACT_ACT_ENVIRONMENT",
+  "Node",
+  "Element",
+  "MutationObserver",
+  "ResizeObserver",
+  "getSelection",
+  "getComputedStyle",
+  "requestAnimationFrame",
+  "cancelAnimationFrame",
+  "KeyboardEvent",
+  "InputEvent",
+  "FocusEvent",
+] as const;
+const saved = keys.map((key) => [key, key in scope, scope[key]] as const);
+let dom: JSDOM;
+let root: Root | undefined;
+let host: HTMLElement;
+
+beforeAll(() => {
+  dom = new JSDOM("<!doctype html><div id=root></div>", { pretendToBeVisual: true, url: "https://cmux.test/" });
+  const { window } = dom;
+  Object.assign(scope, {
+    ResizeObserver: class {
+      observe() {}
+      unobserve() {}
+      disconnect() {}
+    },
+    window,
+    document: window.document,
+    navigator: window.navigator,
+    HTMLElement: window.HTMLElement,
+    IS_REACT_ACT_ENVIRONMENT: true,
+    Element: window.Element,
+    getComputedStyle: window.getComputedStyle.bind(window),
+    requestAnimationFrame: (run: (time: number) => void) => setTimeout(() => run(0), 0),
+    cancelAnimationFrame: (id: number) => clearTimeout(id),
+    KeyboardEvent: window.KeyboardEvent,
+    InputEvent: window.InputEvent,
+    FocusEvent: window.FocusEvent,
+    ...proseMirrorGlobals(window as unknown as Window & typeof globalThis),
+  });
+  window.scrollBy = () => {};
+  const box = () => new window.DOMRect(0, 0, 100, 20);
+  Object.assign(window.Range.prototype, {
+    getClientRects: () => Object.assign([box()], { item: () => box() }),
+    getBoundingClientRect: box,
+  });
+  host = window.document.getElementById("root")!;
+});
+
+afterEach(() => {
+  act(() => root?.unmount());
+  root = undefined;
+  host.replaceChildren();
+});
+
+afterAll(() => {
+  dom.window.close();
+  for (const [key, had, value] of saved) {
+    if (had) scope[key] = value;
+    else delete scope[key];
+  }
+});
+
+const handle = createRef<MarkdownFieldHandle>();
+
+async function render(markdown: string) {
+  const { createRoot } = await import("react-dom/client");
+  const { MarkdownField } = await import("../src/agent-session/acpmux/MarkdownField");
+  root = createRoot(host);
+  await act(async () => root!.render(<MarkdownField ref={handle} value={markdown} onChange={() => {}} />));
+  for (let turn = 0; turn < 200; turn++) {
+    if (host.querySelector("blockquote p")) return;
+    await act(async () => new Promise((resolve) => setTimeout(resolve, 0)));
+  }
+  throw new Error("the quote editor never came up");
+}
+
+async function backspaceAtQuoteStart() {
+  const field = promptField(document);
+  await act(async () => {
+    field.handle.focus();
+    const text = field.handle.text().value;
+    const offset = text.indexOf(field.element.querySelector("blockquote p")!.textContent ?? "");
+    field.handle.writeText(text, offset, offset);
+  });
+  await act(async () => {
+    field.element.dispatchEvent(
+      new window.KeyboardEvent("keydown", { key: "Backspace", code: "Backspace", bubbles: true, cancelable: true }),
+    );
+  });
+  return field;
+}
+
+test("Backspace at the start of a quote turns it into a paragraph, retaining its text", async () => {
+  await render("> Keep this quote text");
+  const field = await backspaceAtQuoteStart();
+  expect(field.element.querySelector("blockquote") === null).toBe(true);
+  expect(field.element.firstElementChild?.tagName).toBe("P");
+  expect(field.value).toBe("Keep this quote text");
+});
+
+test("Backspace in an empty quote removes the quote and leaves a usable paragraph", async () => {
+  await render(">");
+  const field = await backspaceAtQuoteStart();
+  expect(field.element.querySelector("blockquote") === null).toBe(true);
+  expect(field.element.firstElementChild?.tagName).toBe("P");
+  expect(field.value).toBe("");
+});
+
+test("unwrapping a quote after another paragraph does not join or delete either paragraph", async () => {
+  await render("Earlier paragraph\n\n> Keep this quote text");
+  const field = await backspaceAtQuoteStart();
+  expect(field.element.querySelector("blockquote") === null).toBe(true);
+  expect([...field.element.children].map((node) => node.textContent)).toEqual([
+    "Earlier paragraph",
+    "Keep this quote text",
+  ]);
+  expect(field.value).toBe("Earlier paragraph\n\nKeep this quote text");
+});