Fix Form with StartPosition=CenterParent and WindowState=Maximized opening on wrong monitor

[LeafShi1]

Fixes #8932

Root Cause:

In the ShowDialog method, the s_propDialogOwner property was set after CreateControl() was called. This meant that during window creation, when FillInCreateParamsStartPosition is invoked to determine window coordinates, the dialog owner information was not yet available. As a result, the window coordinates defaulted to the primary monitor, and Windows would maximize the form on that monitor regardless of where the parent form was located.

Proposed changes

• Move the Properties.AddOrRemoveValue(s_propDialogOwner, owner) call to before CreateControl() in the ShowDialog method
• Remove the duplicate Properties.AddOrRemoveValue(s_propDialogOwner, owner) call that occurred after CreateControl()
• This allows FillInCreateParamsStartPosition to access the dialog owner information during the CenterScreen positioning logic, which already has code to detect the owner's screen and position the window accordingly

Customer Impact

• Forms with StartPosition=CenterParent and WindowState=Maximized will now correctly maximize on the same monitor as their owner/parent form

Regression?

• Yes

Risk

• Minimal (The change only affects the timing of when s_propDialogOwner is set in Properties dictionary, does not affect the Windows API parent-child relationship)

Screenshots

Before

In the form with value FormWindowState.Maximized it is always displayed on the main monitor.

After

A maximized window is displayed on the monitor where its parent window is located (i.e., the secondary monitor).

AfterChanges.mp4

Test methodology

• Manually

Test environment(s)

• .net 11.0.0-alpha.1.25617.103

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR fixes an issue where modal forms with StartPosition=CenterParent and WindowState=Maximized would incorrectly open on the primary monitor instead of the same monitor as their parent form.

Key Changes:

• Moved the s_propDialogOwner property assignment to occur before CreateControl() is called in the ShowDialog method
• Removed the duplicate property assignment that previously occurred after CreateControl()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.15252%. Comparing base (01eefd4) to head (4347fc0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #14156         +/-   ##
===================================================
+ Coverage   77.15180%   77.15252%   +0.00072%     
===================================================
  Files           3279        3279                 
  Lines         645333      645336          +3     
  Branches       47720       47721          +1     
===================================================
+ Hits          497886      497893          +7     
- Misses        143754      143759          +5     
+ Partials        3693        3684          -9     

Flag	Coverage Δ
Debug	77.15252% <100.00000%> (+0.00072%)	⬆️
integration	18.98369% <100.00000%> (+0.00362%)	⬆️
production	52.01596% <100.00000%> (+0.00189%)	⬆️
test	97.40559% <ø> (ø)
unit	49.43428% <100.00000%> (-0.03317%)	⬇️

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