Fix intermittent deadlock in AnsiTerminal_OutputProgressFrameIsCorrect

[Copilot]

AnsiTerminal_OutputProgressFrameIsCorrect would intermittently hang on macOS due to a deadlock between the background progress refresh timer and the test's synchronization mechanism.

Root Cause

ThreadProc invoked OnProgressStartUpdate while holding _lock. The test subscribes to this event with a blocking WaitOne() to gate timer updates for deterministic output. If the timer fired during a TestCompleted call, the following deadlock occurred:

Thread	State
Timer thread	Holds _lock, blocked on startHandle.WaitOne()
Test thread	Blocked in WriteToTerminal waiting to acquire _lock
Test thread	Never reaches startHandle.Set() → hang

Fix

Move OnProgressStartUpdate?.Invoke(...) to before the lock acquisition in ThreadProc:

// Before
lock (_lock)
{
    OnProgressStartUpdate?.Invoke(this, EventArgs.Empty); // blocks while holding lock
    _terminal.StartUpdate();
    ...
}

// After
OnProgressStartUpdate?.Invoke(this, EventArgs.Empty); // blocks without holding lock
lock (_lock)
{
    _terminal.StartUpdate();
    ...
}

No production behavior change — OnProgressStartUpdate has no subscribers outside tests.

Original prompt

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This section details on the original issue you should resolve

<issue_title>AnsiTerminal_OutputProgressFrameIsCorrect hangs intermittently on macOS</issue_title>
<issue_description>The test hangs intermittently on macOS and usually passes on retry.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes AnsiTerminal_OutputProgressFrameIsCorrect hangs intermittently on macOS #7583

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[Youssef1313]

Clearer description, also from Copilot:

I manually verified that Copilot investigation is correct. This can also be reproduced on Windows by modifying the test to have some Thread.Sleep right after TestExecutionStarted is called in the test method.

What happens then is (same as Copilot investigation but put a little bit differently):

1. TestExecutionStarted is called -> creates a thread that runs ThreadProc, acquires lock and tirggers the event inside the lock.
2. For the event handler to complete, the test needs to reach startHandle.Set()
3. For the test to reach startHandle.Set(), it needs to complete the TestCompleted calls, which need to acquire the same lock held in step 1.

When the test is passing, what happens is that the calls after TestExecutionStarted are scheduled and runs before ThreadProc acquires the lock. That's why adding a sleep causes the hang to be more obvious, as it ensures that ThreadProc acquires the lock first.

[Copilot]

Pull request overview

Fixes an intermittent macOS hang in AnsiTerminal_OutputProgressFrameIsCorrect by avoiding a lock inversion/deadlock between the progress refresh timer thread and test synchronization.

Changes:

• Move OnProgressStartUpdate invocation before _lock acquisition in ThreadProc to prevent blocking while holding _lock.
• Add an explanatory comment documenting the deadlock scenario and rationale.