fix(app): debounce prompt draft persistence

[DGGua]

Issue for this PR

Fixes #36486

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Desktop chat typing lags because every keystroke persists the prompt draft through IPC to electron-store (full-file rewrite), with no debounce. Other inputs do not hit this path.

This adds optional debounceMs to persisted(), enables 300ms debounce for prompt drafts, and updates prompt+cursor in one store write so we do not serialize/persist twice per keystroke. Drafts still flush on idle, blur/unload, and dispose.

How did you verify your code works?

• bun test src/utils/persist.test.ts src/context/prompt-state.test.ts in packages/app
• bun test src/v2/components/prompt-input/store.test.ts in packages/session-ui
• Built and ran Windows desktop package locally; chat input typing felt responsive, and draft restore after quit still worked

Screenshots / recordings

N/A (performance fix, no visual UI change).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[DGGua]

Updated the PR description to the required template and linked Fixes #36486.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[Copilot]

Pull request overview

This PR reduces prompt draft persistence overhead during typing (especially on desktop) by debouncing storage writes and minimizing redundant persisted-store updates, with a unit test intended to validate write coalescing.

Changes:

• Add an optional debounceMs to persisted targets and wrap storage with a debounced write layer.
• Update prompt + cursor in a single store write to avoid duplicate persistence serialization.
• Add a unit test to validate debounced write coalescing behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
packages/session-ui/src/v2/components/prompt-input/store.ts	Consolidates prompt+cursor updates into single store writes for fewer persisted-store updates.
packages/app/src/utils/persist.ts	Introduces debounceMs and debounced storage wrapper for persisted writes.
packages/app/src/utils/persist.test.ts	Adds coverage for debounced storage write coalescing.
packages/app/src/context/prompt-state.ts	Applies debounced persistence to prompt drafts and consolidates prompt+cursor writes.

Suppressed comments (1)

packages/app/src/utils/persist.test.ts:234

• The debounce test currently relies on wall-clock timing and a Promise wrapper. Using vi.useFakeTimers() makes this deterministic and also lets the test cover the important cleanup-flush behavior (dispose before the timer fires) that is relied on when quitting the desktop app soon after typing.

  test("debounceWrites coalesces rapid setItem calls into one flush", async () => {
    await new Promise<void>((resolve, reject) => {
      createRoot((dispose) => {
        try {
          const api = persistTesting.debounceWrites(persistTesting.localStorageDirect(), 20)
          api.setItem("draft", '{"v":1}')
          api.setItem("draft", '{"v":2}')
          api.setItem("draft", '{"v":3}')
          expect(storage.calls.set).toBe(0)

          setTimeout(() => {
            try {
              expect(storage.calls.set).toBe(1)
              expect(storage.getItem("draft")).toBe('{"v":3}')
              dispose()
              resolve()
            } catch (error) {
              dispose()
              reject(error)
            }
          }, 40)
        } catch (error) {

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[DGGua]

Addressed Copilot review feedback:

• debounceWrites now wraps storage with a Proxy so getter-based fields like desktop AsyncStorage.length stay live
• debounce tests use Bun fake timers (no wall-clock sleeps) and cover dispose flush + length getter preservation