Handle buffer changes made by a handler of the OnIdle event

[daxian-dbw]

PR Summary

Fix #3939

Handle buffer changes made by a handler of the OnIdle event.
We don't update the initial cursor top when buffer was changed by an event handler.

It requires interaction with PowerShell to test out this change, and thus I cannot add an automation test.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure you've added one or more new tests
• Make sure you've tested these changes in terminals that PowerShell is commonly used in (i.e. conhost.exe, Windows Terminal, Visual Studio Code Integrated Terminal, etc.)
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed at PowerShell-Docs
    • Doc Issue filed:

Microsoft Reviewers: Open in CodeFlow