Ending screen sharing during extension startup starts a microphone-only recording #3167

Description

Ending screen sharing before the extension finishes starting can still begin a microphone-only recording. The recording badge stays active after screen capture has ended, and pressing Stop sends an audio-only file through the video-upload workflow.

Reproduction

1. Enable the microphone and the default three-second countdown.
2. Start a screen recording.
3. End screen sharing before the countdown finishes.
4. Observe that the extension enters recording anyway, then press Stop.

Expected: cancel the pending start, return to idle and release the microphone.

Actual: recording begins without live video and continues capturing microphone audio until stopped.

The same failure occurs if capture ends while the recording-creation request is pending.

Additional Context

Baseline main: d1f8017686092e0ee3bb20ff7837d2fd8176f6d5. Affected source is unchanged from discovery baseline 8d40aaf98e1adccd5ee208353ea68c5552e14509.

The video-track ended listener is installed after asynchronous setup. An earlier ended event is not replayed. A live mixed microphone track lets MediaRecorder start successfully.

An actual-module Chromium fixture with real native tracks, MediaRecorder, AudioContext, IndexedDB and encoding reproduces a 499-byte Opus-only upload on baseline. The focused correction cancels, stops the microphone and uploads nothing. Acquisition, extension IPC and HTTP are fixture boundaries; physical capture and a packaged extension were not exercised.

Three focused startup regressions fail on baseline and all nine recorder checks pass after the correction, including existing microphone modes and source ending after startup.