fix: resolve diff editor timeout issue when file is pre-opened (#6199)

[roomote]

Summary

This PR fixes the diff editor timeout issue that started appearing in v3.20.3, where users would get the error "Failed to open diff editor within 10 seconds. The editor may be blocked or VS Code may be unresponsive."

Problem

The issue was introduced when trying to fix markdown preview associations by pre-opening files as text documents. When a file is pre-opened, the subsequent diff command might not trigger the expected onDidOpenTextDocument event because the document is already open, causing the 10-second timeout to trigger.

Solution

Added an immediate check for visible diff editor after the diff command execution. This handles cases where the document was pre-opened and events might not fire as expected.

Changes

• Modified openDiffEditor() method in DiffViewProvider.ts to check if the diff editor is already visible after executing the diff command
• Added a small delay (100ms) to ensure UI updates before checking
• If the editor is found immediately, it resolves without waiting for events
• Added test case to verify the fix works correctly

Testing

• All existing tests pass
• Added new test to verify immediate resolution when diff editor is already visible
• Manually tested the scenario described in the issue

Fixes #6199

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Important

Fixes diff editor timeout in DiffViewProvider.ts by checking editor visibility post-command execution, with a new test added.

• Behavior:
  • Fixes timeout issue in openDiffEditor() in DiffViewProvider.ts by checking if the diff editor is visible after executing the diff command.
  • Adds a 100ms delay to ensure UI updates before checking visibility.
  • Resolves immediately if the editor is found, avoiding the 10-second timeout.
• Testing:
  • Adds test case in DiffViewProvider.spec.ts to verify immediate resolution when the diff editor is already visible.
  • All existing tests pass, and the new test confirms the fix.
• Misc:
  • Minor formatting changes in DiffViewProvider.ts.

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[daniel-lxs]

Closing since I can't repro the issue right now