Add AppContext switch to mirror Owner.TopMost for modal dialogs (restore .NET Framework‑like behavior)

[LeafShi1]

Fixes #14101

Proposed changes

• Add an switch System.Windows.Forms.MirrorTopMostForModalDialogs.
  When enabled, ShowDialog will temporarily set the dialog as TopMost if its owner is TopMost, and restore the original state after closing.
  Default is false to keep current behavior.

Customer Impact

• With the switch off: No change, modern .NET behavior remains.
• With the switch on: Modal dialogs owned by TopMost forms will stay above the owner during the modal session, matching .NET Framework behavior.

Regression?

• Yes

Risk

• Minimal

Screenshots

Before

If the owner is TopMost and the dialog is not, the dialog can be pushed behind the owner during the modal session.

After

Modal dialogs temporarily become TopMost when the owner is TopMost, and stay above the owner until closed.

Test methodology

• Manually

Test environment(s)

• .net 10.0.0-rc.3.25603.106

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR adds an AppContext switch System.Windows.Forms.MirrorTopMostForModalDialogs to restore .NET Framework-like behavior where modal dialogs temporarily inherit the TopMost property from their owner Form during the modal session.

Key Changes:

• Introduces a new AppContext switch that when enabled, automatically sets TopMost=true on modal dialogs if their owner Form has TopMost=true
• Restores the original TopMost state after the dialog closes
• Default behavior is false to maintain current .NET behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/System.Windows.Forms.Primitives/src/System/LocalAppContextSwitches/LocalAppContextSwitches.cs	Adds the MirrorTopMostForModalDialogs switch definition and property
src/System.Windows.Forms/System/Windows/Forms/Form.cs	Implements the TopMost mirroring logic in ShowDialog method, capturing original state and restoring it after the modal session

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.15426%. Comparing base (5d64bba) to head (8ff448e).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #14138         +/-   ##
===================================================
+ Coverage   76.01271%   77.15426%   +1.14154%     
===================================================
  Files           3279        3279                 
  Lines         645325      645337         +12     
  Branches       47720       47722          +2     
===================================================
+ Hits          490529      497905       +7376     
+ Misses        145491      143738       -1753     
+ Partials        9305        3694       -5611     

Flag	Coverage Δ
Debug	77.15426% <25.00000%> (+1.14154%)	⬆️
integration	18.98449% <25.00000%> (?)
production	52.01885% <25.00000%> (+2.55940%)	⬆️
test	97.40749% <ø> (ø)
unit	49.47059% <25.00000%> (+0.01114%)	⬆️

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