Redraw issues in VLC and Virtualbox and Qt 5.15.6

Since commit 290b405872602de931646fe4f769eff208f9bbef (introduced in 5.15.5 in qt5 qtbase module), VLC and Virtualbox have redraw issues on some widgets.

In VLC, the reproduction steps are:

1. Open VLC
2. Open the playlist (CTRL + L)
3. Play a video
4. Stop the video
5. While the playlist is open, resize the window, the playlist is not redrawn and visual glitches appear. The playlist is redrawn only when the size of the window is reduced.
6. Play the video again, the screen is kept black (instead of displaying the video)

In Virtualbox, the reproduction steps are:

1. Open the snapshots view (click on the menu button on a VM, then select Snapshots)
2. Then switch between tabs "Attributes" and "Information"
3. While the attributes tab is open, resize the window, there are visual glitches inside the "Description" text area.

See these videos:

Before 290b405872602de931646fe4f769eff208f9bbef (no glitches):

qt_5.15.4+dfsg-2_vlc_no_glitches-1.mp4

After:

qt_5.15.6+dfsg-2_vlc_glitches.mp4

qt_5.15.6+dfsg-2_virtualbox_glitches.mp4

To find the commit, I've bisected qtbase git repository. This was initially detect on Debian Unstable, see details here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1023580.

The bug is probably already fixed in 5.15.10, as the offending commit was reverted because of QTBUG-99810 and QTBUG-101347.

But I've created this bug anyway because the issue here is not about undocked windows or menus, but common widgets within a window and affecting popular applications (VLC and Virtualbox at least).