Debug weird UI behavior with CDP logging

Debug weird UI behavior with CDP logging

Use this when the app does something weird and the behavior must be observed live: refreshes, remounts, flickers, rows briefly unfolding, wrong ordering for a moment, state changing then reverting, actions firing twice, stale UI, unexpected navigation, focus loss, scroll jumps, or anything where code inspection alone is likely to lie.

Rules

• Reproduce in the running dev app. Do not guess from code only.
• Use CDP against Electron (CDP_PORT=39217) when possible.
• Instrument the page with a small in-page logger before performing the action.
• Only act on disposable data.
  • Create temp branches/worktrees/projects/sessions when needed.
  • Use obvious names like cdp-debug-* or tmp-*.
  • Delete/clean them up after the test.
  • Do not use important branches, real user sessions, or serious project data for destructive actions.
• If a destructive action is required, make the temporary object first, verify it appears, then delete that object only.

CDP setup

List targets:

CDP_PORT=39217 node /home/igorw/.pi/agent/codex-conversion-custom-tools/browser/cdp.mjs list

Use the target id prefix from list in later commands.

Install a DOM mutation logger

Adapt selectors to the surface being debugged. For sidebar branch/worktree flicker, this logger captures row labels, expanded state, and scroll position:

CDP_PORT=39217 node /home/igorw/.pi/agent/codex-conversion-custom-tools/browser/cdp.mjs eval <target> '(() => {
  window.__uiDebug?.observer?.disconnect?.()
  const root = document.querySelector(".sidebar-project-work-section")
  window.__uiDebug = { events: [], startedAt: Date.now() }
  const labels = () => Array.from(document.querySelectorAll(".sidebar-project-work-branch-toggle"))
    .map((el) => ({
      text: el.textContent.trim(),
      expanded: el.getAttribute("aria-expanded"),
      disabled: el.hasAttribute("disabled"),
    }))
  const snap = (reason) => window.__uiDebug.events.push({
    t: Date.now(),
    dt: Date.now() - window.__uiDebug.startedAt,
    reason,
    labels: labels(),
    scrollTop: document.querySelector(".sidebar-project-work-scroll-shell")?.scrollTop ?? null,
  })
  snap("install")
  const observer = new MutationObserver((mutations) => {
    if (!mutations.some((m) => m.type === "childList" || m.type === "attributes")) return
    snap("mutation:" + mutations.map((m) =>
      m.type + (m.attributeName ? ":" + m.attributeName : "") + ":+" + m.addedNodes.length + ":-" + m.removedNodes.length
    ).slice(0, 8).join(","))
  })
  if (root) observer.observe(root, {
    subtree: true,
    childList: true,
    attributes: true,
    attributeFilter: ["aria-expanded", "data-current", "data-divider-after", "data-divider-before"],
  })
  window.__uiDebug.observer = observer
  return labels().slice(0, 20)
})()'

Reproduce with temp data

Example pattern:

1. Create a temp thing (cdp-debug-*).
2. Confirm it appears.
3. Add a manual marker before the suspect action:

CDP_PORT=39217 node /home/igorw/.pi/agent/codex-conversion-custom-tools/browser/cdp.mjs eval <target> '(() => {
  window.__uiDebug.events.push({ reason: "before-action", t: Date.now(), dt: Date.now() - window.__uiDebug.startedAt })
  return true
})()'

4. Perform the suspect action.
5. Dump recent events:

CDP_PORT=39217 node /home/igorw/.pi/agent/codex-conversion-custom-tools/browser/cdp.mjs eval <target> '(() => window.__uiDebug?.events?.slice(-80))()'

What to look for

• Full page reload:
  • CDP state disappears, window.__uiDebug is gone, or Vite logs full reload.
  • Check dev server watcher config, especially generated folders/worktrees.
• Real component/state flicker:
  • Logger persists and shows multiple DOM mutations.
  • Compare snapshots before/after each mutation.
  • Look for stale mixed sources of truth: cache patched before related query/state refreshed, or vice versa.
• Remount/state reset:
  • Local UI state resets while no page reload happened.
  • Check keys, conditional rendering, parent state sync effects, and broad query writes.

Fix strategy

• Prefer one consistent state transition over optimistic patch + stale secondary state.
• If UI derives from two sources, update them in an order that avoids impossible intermediate states.
• Preserve object references on broad cache merges when data is unchanged.
• Avoid full-shell/query refreshes for localized mutations.
• Add a regression test for the specific ordering/reference behavior when practical.

Cleanup

• Remove temp branches/worktrees/projects/sessions created for the test.
• Re-run the logger after the fix with a fresh temp object.
• Keep unrelated local changes unstaged.
• Run the repo verification command when code changed.