fix(quick-edit): stop RELOAD storms from cross-block index drift

WORKLOG.md

@@ -2,6 +2,42 @@
 
 ## 2026-09-07
 
+### quick-edit — stop RELOAD storms from cross-block index drift
+
+Debugged via da-live's `ew-editor-doc` collab-diagnostics branch (multi-user
+test showed a collaborator's continuous edits pegging the main thread for
+5-6s at a stretch, blocking local typing). Root cause traced to
+`nx/public/plugins/quick-edit/src/prose.js`'s `createEditor`: it looked up
+its target block by an exact `data-prose-index` match, but that index is a
+global ProseMirror position — any edit *before* a block shifts it.
+`handleTransaction` already re-shifts every other block's index for edits
+inside an already-open mini-editor (`updateInstrumentation`), but
+`createEditor` — the path taken the first time a block is touched by a
+*remote* edit — never did, so the first remote edit to any not-yet-opened
+block left every later block's cached index stale. Eventually some block's
+`SET_EDITOR_STATE` arrived with a `cursorOffset` matching nothing, and the
+portal gave up and asked the host to `RELOAD` (full body resend), which the
+host answered unconditionally — no debounce — so a sustained editing burst
+from one collaborator could retrigger this indefinitely.
+
+Fixed `createEditor` to fall back to `findTextBlock`'s existing
+nearest-indexed-block lookup (`dom-index.js`) instead of giving up — the same
+drift-tolerant match `findImageAtProseIndex` already relies on for images.
+Added an `exclude` param to `findTextBlock`/`findNearestIndexed` so the
+fallback can't resolve to (and destructively replace) a *different* block's
+already-open `.prosemirror-editor`; the remote-cursor collaborator badge is
+now only copied across on an exact match, not the fallback, so it can't get
+misattributed to the wrong paragraph. Da-live also got a `quick-edit-controller.js`
+RELOAD-coalescing debounce (150ms) as a stopgap while this was tracked down;
+kept, since it's still a legitimate backstop.
+
+Verified via a two-browser test (da-nx files served through Chrome local
+overrides): the RELOAD storm is gone under sustained multi-user editing.
+
+Review follow-ups: normalized `cursorOffset` to `Number` in `createEditor` so
+the exact-match badge gate can't silently fail on a string, and added unit
+tests for `findTextBlock`'s exclude + nearest-block fallback.
+
 ### nx2/blocks/editortoggle — stop implicit `nx2:ew-user-enabled` writes on navigation
 
 `connectedCallback` used to reconcile the persisted `nx2:ew-user-enabled` flag

nx/public/plugins/quick-edit/src/dom-index.js

@@ -12,12 +12,13 @@ export function safeQuerySelectorAll(root, selector) {
   }
 }
 
-function findNearestIndexed(attr, from, root) {
-  const exact = root.querySelector(`[${attr}="${from}"]`);
+function findNearestIndexed(attr, from, root, exclude) {
+  const suffix = exclude ? `:not(${exclude})` : '';
+  const exact = root.querySelector(`[${attr}="${from}"]${suffix}`);
   if (exact) return exact;
   let best = null;
   let bestIndex = -1;
-  root.querySelectorAll(`[${attr}]`).forEach((el) => {
+  root.querySelectorAll(`[${attr}]${suffix}`).forEach((el) => {
     const idx = parseIndex(el.getAttribute(attr));
     if (idx == null || idx > from) return;
     if (idx > bestIndex) {
@@ -28,8 +29,10 @@ function findNearestIndexed(attr, from, root) {
   return best;
 }
 
-export function findTextBlock(from, root = document) {
-  return findNearestIndexed('data-prose-index', from, root);
+// exclude keeps an already-open editor out of the nearest-match fallback, so a
+// drifted cursorOffset can't resolve to and replace a different block's editor.
+export function findTextBlock(from, root = document, exclude = null) {
+  return findNearestIndexed('data-prose-index', from, root, exclude);
 }
 
 export function findBlock(from, root = document) {

nx/public/plugins/quick-edit/src/prose.js

@@ -13,6 +13,7 @@ import { createSimpleKeymap } from './simple-keymap.js';
 import { createImageWrapperPlugin } from './image-wrapper.js';
 import { setupImageDropListeners } from './images.js';
 import { setRemoteCursors } from './cursors.js';
+import { findTextBlock } from './dom-index.js';
 import { MESSAGE_TYPES } from '../../../../utils/message-types.js';
 
 function marksEqual(a, b) {
@@ -167,6 +168,9 @@ function keydown(view, event) {
 }
 
 function createEditor(cursorOffset, state, ctx) {
+  // Normalize once: the exact-match badge gate below is a strict === and would
+  // silently never match if cursorOffset arrived as a string.
+  const offset = Number(cursorOffset);
   const schema = getSchema();
   const node = schema.nodeFromJSON(state);
   const doc = schema.node('doc', null, [node]);
@@ -178,17 +182,22 @@ function createEditor(cursorOffset, state, ctx) {
   });
 
   const editorParent = document.createElement('div');
-  editorParent.setAttribute('data-prose-index', cursorOffset);
+  editorParent.setAttribute('data-prose-index', offset);
   editorParent.classList.add('prosemirror-editor');
 
-  const element = document.querySelector(`[data-prose-index="${cursorOffset}"]`);
+  // Drift-tolerant lookup: an exact match can miss after another block's remote edit
+  // shifts positions. Exclude open editors so the fallback can't steal a live one.
+  const element = findTextBlock(offset, document, '.prosemirror-editor');
 
   if (!element) {
     ctx.port.postMessage({ type: MESSAGE_TYPES.RELOAD });
     return;
   }
 
-  if (element.getAttribute('data-cursor-remote')) {
+  // Only trust the found element's remote-cursor badge on an exact match — on the
+  // nearest-block fallback it belongs to whatever block drift landed on, not this one.
+  const isExactMatch = parseInt(element.getAttribute('data-prose-index'), 10) === offset;
+  if (isExactMatch && element.getAttribute('data-cursor-remote')) {
     editorParent.setAttribute('data-cursor-remote', element.getAttribute('data-cursor-remote'));
     editorParent.setAttribute('data-cursor-remote-color', element.getAttribute('data-cursor-remote-color'));
   }

test/nx/public/plugins/quick-edit/dom-index.test.js

@@ -1,5 +1,38 @@
 import { expect } from '@esm-bundle/chai';
-import { restoreBlockIndices } from '../../../../../nx/public/plugins/quick-edit/src/dom-index.js';
+import { restoreBlockIndices, findTextBlock } from '../../../../../nx/public/plugins/quick-edit/src/dom-index.js';
+
+describe('findTextBlock', () => {
+  function root(html) {
+    const el = document.createElement('div');
+    el.innerHTML = html;
+    return el;
+  }
+
+  it('returns the exact data-prose-index match', () => {
+    const r = root('<p data-prose-index="10">a</p><p data-prose-index="20">b</p>');
+    expect(findTextBlock(20, r)).to.equal(r.querySelector('[data-prose-index="20"]'));
+  });
+
+  it('falls back to the nearest block at-or-before a drifted offset', () => {
+    const r = root('<p data-prose-index="10">a</p><p data-prose-index="20">b</p>');
+    // 23 has no exact match (positions drifted) — nearest at-or-before is 20.
+    expect(findTextBlock(23, r)).to.equal(r.querySelector('[data-prose-index="20"]'));
+  });
+
+  it('excludes an already-open editor from an exact match', () => {
+    const r = root('<div class="prosemirror-editor" data-prose-index="20">live</div>'
+      + '<p data-prose-index="10">a</p>');
+    // 20 matches the live editor exactly, but exclude must skip it and fall back to 10.
+    expect(findTextBlock(20, r, '.prosemirror-editor')).to.equal(r.querySelector('p'));
+  });
+
+  it('excludes an already-open editor from the nearest-block fallback', () => {
+    const r = root('<p data-prose-index="10">a</p>'
+      + '<div class="prosemirror-editor" data-prose-index="20">live</div>');
+    // Nearest at-or-before 23 is the live editor at 20; exclude it and use 10.
+    expect(findTextBlock(23, r, '.prosemirror-editor')).to.equal(r.querySelector('p'));
+  });
+});
 
 describe('restoreBlockIndices', () => {
   it('stamps the authored variant from the source onto the live block', () => {