[playwright] [Bug]: Firefox headed hangs indefinitely on page.setViewportSize()

## Observed symptom [Bug]: Firefox headed hangs indefinitely on page.setViewportSize() ### Version 1.64.0-alpha-2026-09-30 ### Steps to reproduce 1. Clone https://github.com/cccplex/playwright-firefox-setviewport-hang 2. `cd playwright-firefox-setviewport-hang` 3. `npm install` 4. `npx playwright install firefox` 5. `node repro.js` in a graphical desktop session 6. Firefox starts in headed mode. 7. The script prints `Starting setViewportSize...` and does not return. 8. After 20 seconds the script exits with code 124. ### Expected behavior `page.setViewportSize({ width: 400, height: 800 })` should return and the page viewport should become 400×800. The same operation works in headless Firefox, and headed Firefox works when the viewport is specified when creating the Brows ## Literal error text ``` cd playwright-firefox-setviewport-hang ``` ## Environment GitHub microsoft/playwright #43028 ## Trigger / context ### Version 1.64.0-alpha-2026-09-30 ### Steps to reproduce 1. Clone https://github.com/cccplex/playwright-firefox-setviewport-hang 2. `cd playwright-firefox-setviewport-hang` 3. `npm install` 4. `npx playwright install firefox` 5. `node repro.js` in a graphical desktop session 6. Firefox starts in headed mode. 7. The script prints `Starting setViewportSize...` and does not return. 8. After 20 seconds the script exits with code 124. ### Expected behavior `page.setViewportSize({ width: 400, height: 800 })` should return and the page viewport should become 400×800. The same operation works in headless Firefox, and headed Firefox works when the viewport is specified when creating the BrowserContext. ### Actual behavior In headed Firefox, `page.setViewportSize({ width: 400, height: 800 })` does not return. The call remains pending until the reproduction's 20-second timeout terminates ## Source evidence - https://github.com/microsoft/playwright/issues/43028 (2026-09-30T23:14:00Z) — #43028 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:52:00.922591Z - scout_id: C7aa7ac67f5 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.