[Bug 57373] New: regression: dragged child windows partially disappear

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=57373

Bug ID: 57373
           Summary: regression: dragged child windows partially disappear
           Product: Wine
           Version: 9.20
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: -unknown
          Assignee: [e-mail]
          Reporter: [e-mail]
      Distribution: ---

Created attachment 77341
  --> https://bugs.winehq.org/attachment.cgi?id=77341
video showing the bug

In Wine 9.20, dragging a child window does not re-draw the child window
contents or decoration in its new location, causing much of the window to be
invisible or misaligned from its actual location.

Steps to reproduce:

1. You will need an application that uses child windows. Two free audio apps
that I use that exhibit this behavior are Sekaiju and Wavosaur. These repro
steps will use Sekaiju, which you can download here:
https://openmidiproject.opal.ne.jp/Sekaiju_en.html
2. Start Sekaiju.exe. It will complain about not being able to find the
Microsoft GS Wavetable Synth. Click "OK".
3. Drag the "Untitled1(Track List)" child window to another location within the
main window.

Result: The child window is not moved, and part of the child window's contents
are still shown in their old location.

Expected result: The child window should now appear in its new location.

Test systems: I was able to reproduce this bug using the winehq-devel packages
for Ubuntu Noble in KDE neon (desktop using X11, laptop using Wayland) and
Xubuntu (Virtualbox using X11).

Workaround: downgrade to Wine 9.19.