QMenu/QComboBox border-radius not properly rendered on Windows

   Steps to reproduce:
   1. Create a QComboBox/QMenu
   2. Set border-radius in stylesheet
   3. Run on Windows 11

   Expected behavior:
   - Rounded corners should be rendered properly
   - No square shadow/artifacts in corners

   Actual behavior:
   - Square artifacts appear in the bottom-right corner
   - Border-radius is not properly rendered

Additional important observation:

The border-radius behavior during popup animation is problematic:

1. Animation sequence:
   - When popup starts: ALL corners are square
   - During animation: ALL corners remain square
   - After animation completes: 
     * Three corners suddenly become rounded
     * Bottom-right corner remains square

2. UX Issues:
   - Visual discontinuity: Sudden change from square to rounded corners
   - Breaks animation fluidity
   - Appears unpolished and buggy
   - Poor user experience even if bottom-right corner issue is fixed

3. Expected behavior:
   - Corners should be rounded from the start of animation
   - Maintain consistent rounded appearance throughout the animation
   - Smooth and professional looking transition

This animation behavior is a separate issue from the bottom-right corner problem, and both need to be addressed for a proper modern UI experience.