fix: Hide icon when ShowIcon = false

[RussKie]

Fixes #3022

Proposed changes

• Correct a regression introduced in Cleanup Form.IsRestrictedWindow #695 when a condition was flipped that stopped setting form's extended style flag WS_EX_DLGMODALFRAME.

Customer Impact

Customers now able to hide form's icon at runtime by setting myform.ShowIcon = false.
Workarounds involve either

• setting an icon on a form to a transparent blank icon, or
• performing an interop call to hide the icon before hiding the icon:

  if (myform.ShowIcon)
  {
      int extendedStyle = (int)User32.GetWindowLong(myform.Handle, User32.GWL.EXSTYLE);
      User32.SetWindowLong(myform.Handle, User32.GWL.EXSTYLE, (IntPtr)(extendedStyle | (int)User32.WS_EX.DLGMODALFRAME));
  }
  
  myform.ShowIcon =false;

Regression?

• Yes, introduced in .NET Core 3.0

Risk

• Minimal

Test methodology

• manual tests in the integration test app
• unit tests

Microsoft Reviewers: Open in CodeFlow

[merriemcgaw]

Let's mark this a known issue for 3.1 including the workaround.

[codecov]

Codecov Report

Merging #3055 into release/3.1 will increase coverage by 0.19747%.
The diff coverage is 100.00000%.

@@                  Coverage Diff                  @@
##           release/3.1       #3055         +/-   ##
=====================================================
+ Coverage     24.88340%   25.08087%   +0.19746%     
=====================================================
  Files              844         844                 
  Lines           260290      260310         +20     
  Branches         36896       36904          +8     
=====================================================
+ Hits             64769       65288        +519     
+ Misses          190807      190240        -567     
- Partials          4714        4782         +68     

Flag	Coverage Δ
#Debug	25.08087% <100.00000%> (+0.19746%)	⬆️
#production	25.08087% <100.00000%> (+0.19746%)	⬆️
#test	?

[codecov]

Codecov Report

Merging #3055 into release/3.1 will increase coverage by 0.27660%.
The diff coverage is 100.00000%.

@@                  Coverage Diff                  @@
##           release/3.1       #3055         +/-   ##
=====================================================
+ Coverage     24.88340%   25.16000%   +0.27659%     
=====================================================
  Files              844         844                 
  Lines           260290      260310         +20     
  Branches         36896       36904          +8     
=====================================================
+ Hits             64769       65494        +725     
+ Misses          190807      190052        -755     
- Partials          4714        4764         +50     

Flag	Coverage Δ
#Debug	25.16000% <100.00000%> (+0.27659%)	⬆️
#production	25.16000% <100.00000%> (+0.27659%)	⬆️
#test	?