Issue 2053861 fix combo box flickering issue

[LeafShi1]

Fixes #2053861

Proposed changes

• Separate DrawPopUpCombo from DrawFlatCombo method，draw the outer black border only when the ComboBox mouse is over

Customer Impact

• Combobox no longer flickers when hovering over it

Regression?

• No

Risk

• Minimal

Screenshots

Before

Combo box flashes when you move the mouse over it

After

Combo boxes no longer flicker when you move the mouse over them

Test methodology

• Manually

Test environment(s)

• .net 9.0.0-preview.6.24305.2

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 9.37500% with 29 lines in your changes missing coverage. Please review.

Project coverage is 74.46985%. Comparing base (fc56416) to head (86889e0). Report is 42 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #11529         +/-   ##
===================================================
+ Coverage   74.39859%   74.46985%   +0.07125%     
===================================================
  Files           3032        3039          +7     
  Lines         628149      629068        +919     
  Branches       46828       46836          +8     
===================================================
+ Hits          467334      468466       +1132     
+ Misses        157465      157242        -223     
- Partials        3350        3360         +10     

Flag	Coverage Δ
Debug	74.46985% <9.37500%> (+0.07125%)	:arrow_up:
integration	17.97590% <0.00000%> (-0.02466%)	:arrow_down:
production	47.32729% <9.37500%> (+0.08032%)	:arrow_up:
test	96.96223% <ø> (-0.00449%)	:arrow_down:
unit	44.31360% <9.37500%> (+0.08465%)	:arrow_up:

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[Tanya-Solyanik]

This link - https://devdiv.visualstudio.com/DevDiv/_queries/edit/2053861/?queryId=7c94b3b5-6b84-4f3d-9f76-ff1a7a7b7581 does not work, please fix it.

Had you tested this change at different themes and RtL applications?

The change looks reasonable.

[LeafShi1]

This link - https://devdiv.visualstudio.com/DevDiv/_queries/edit/2053861/?queryId=7c94b3b5-6b84-4f3d-9f76-ff1a7a7b7581 does not work, please fix it.

Had you tested this change at different themes and RtL applications?

The change looks reasonable.

Updated the link of the description and test in different environment Test in RTL app:

Test in different theme:

Test in Flat Style:

There are still a flickering in Flat Style, and this flickering issue can be resolved by remove https://github.com/dotnet/winforms/blob/889fca6df2aa684e3557ef7737b443ed3af32455/src/System.Windows.Forms/src/System/Windows/Forms/Controls/ComboBox/ComboBox.cs#L3880

But I am not sure if we can remove this directly, @Tanya-Solyanik What do you think?

[Tanya-Solyanik]

@LeafShi1 - How do the high contrast themes look before your changes, did it get better or worse with the fix? FYI @merriemcgaw

[LeafShi1]

Before the modification, the Popup state was not obvious, and only a black border was displayed when the mouse entered the control

[merriemcgaw]

This is definitely not worse from my perspective. @Tanya-Solyanik?

[Tanya-Solyanik]

This is definitely not worse from my perspective. @Tanya-Solyanik?

The new behavior looks more intentional, i.e. we are adding a border when mouse is hovering. I don't mind taking this fix.

[Tanya-Solyanik]

@Olina-Zhang - could you please test this fix?

[Tanya-Solyanik]

@LeafShi1

But I am not sure if we can remove this directly, @Tanya-Solyanik What do you think?

No, we shouldn't do that. I agree. To fix that we would have to redesign the control look, maybe add a prominent border in the state when the mouse is not above the control

[LeafShi1]

@LeafShi1

But I am not sure if we can remove this directly, @Tanya-Solyanik What do you think?

No, we shouldn't do that. I agree. To fix that we would have to redesign the control look, maybe add a prominent border in the state when the mouse is not above the control

@Tanya-Solyanik Do we need to redesign the controls appearance now?

Before drawing Flat, the combobox control is as follows. Pic1:

The method of drawing Flat is to draw Outboder and innerBorder. The result after drawing is as follows Pic2:

When the cursor enters, it is necessary to complete the drawing from Pic1 to Pic2，the actual cause of the flickering is the use of g.DrawRectangle four times in a row when drawing the Flat type. So adding a protruding border when the mouse is not over the control does not seem to solve the flickering problem

[Tanya-Solyanik]

@Tanya-Solyanik Do we need to redesign the controls appearance now?

No

[Syareel-Sukeri]

Verified this issue on 9.0.100-preview.7.24371.4 with dlls built from winforms repo of main branch, it was fixed. Flickering effect is no longer there when hovering over ComboBox. Results are as below:

PopupStyle

https://github.com/user-attachments/assets/fff6c129-6890-4203-9618-4ac17b8dfcab

FlatStyle

https://github.com/user-attachments/assets/eaaf3e8c-c057-4076-b47d-5f294e5ba9e9

Different Themes

https://github.com/user-attachments/assets/4c248269-c303-4a0b-a790-faf412703505

https://github.com/user-attachments/assets/3c7111d8-e452-49f8-ba89-080e463f9ae4

https://github.com/user-attachments/assets/bd6a23f1-5c09-4b9a-9d9e-69c7ebce47a7

https://github.com/user-attachments/assets/22d15301-8eda-4926-b7a4-d2abcf8949f2

[Olina-Zhang]

Verified this PR fixing in .NET 9.0.100-preview7.24402.8 test pass build from provided VS VAL build, it was fixed for ComboBox flickering issue when setting this control's FlatStyle == PopUp. But still repro for ComboBox's FlatStyle == Flat case. These are same as the testing result we tested before.

https://github.com/user-attachments/assets/b85553d6-0cdb-43a4-919c-3e25781be169