Custom WinForms ComboBox Flickers in Windows 11

[jmiles17]

I work on a Visual Studio extension.
I have been testing the extension on version 17.13 of Visual Studio with .NET Framework version 4.8

I have a custom WinForms ComboBox which I repaint by intercepting WM_PAINT in WndProc.
I first call base.WndProc.
Then I paint the background, border and dropdown button, and when the ComboBox is not editable, I also paint the text.

I also handle OnMouseEnter and OnMouseLeave, but I merely set a private '_isHovered' variable and then call the base function, and I do not call Invalidate()

I am also intercepting wm_ctlcoloredit and wm_ctlcolorstatic, but those do not seem related to this.

On Windows 10, the control works reasonably well and there is no flickering.
On Windows 11, when I hover over the control, it flickers a fair amount, especially when I hover back and forth near the dropdown button.

I have tried the following:
- Intercept wm_erasebkgnd
○ This seems to help a little, but not much.
- Set UserPaint, OptimizedDoubleBuffer, AllPaintingInWmPaint
○ This did not seem to help, and I also seem unable/do not know how to render the text correctly for an editable ComboBox
- Override CreateParams and set ws_ex_composited
○ I did this in the custom ComboBox itself, not the form, because it is just the specific hovered control that is flickering
○ This causes an unknown error

I have tried some other small changes too.

Do you have any advice/know what might be causing this issue?

Also, I see this issue: #12739
but it does not seem entirely relevant.

Thanks.

[jmiles17]

I posted a sample application with the combobox here:
https://github.com/jmiles17/winforms_combobox_sample