Bug 514812

Created attachment 188666 [details]
Screenshot of KDiff3 with disabled UI and black editor panels

SUMMARY
When I resize a window in KDiff3, the text views turn black, the entire UI is disabled, and there is a grayed-out progress bar and Cancel button in the status bar.

STEPS TO REPRODUCE
1. Open KDiff3 in a Plasma Wayland session.
2. Load or paste one or more files.
3. Resize the window quickly.

Maximizing or unmaximizing the app once at a time, or resizing the window by exactly one pixel at a time, is safe. Any resize operation disables the UI, turns the text views white, and adds a progress bar and grayed-out Cancel button to the status bar, until the repainting finishes. If you send a second resize event before the window finishes repainting, the text views turn black and the UI never gets reenabled.

I think this is an issue where under Wayland, the app receives events and triggers repaints while the previous resize operation is still in progress, and this reentrant repaint sends the app into a broken state unable to finish redrawing.

This issue does not happen in `QT_QPA_PLATFORM=xcb kdiff3`, which runs under XWayland.

OBSERVED RESULT
The app hangs in an unusable state.

EXPECTED RESULT
The app does not hang.

SOFTWARE/OS VERSIONS
Operating System: Fedora Linux 43
KDE Plasma Version: 6.5.5
KDE Frameworks Version: 6.22.0
Qt Version: 6.10.1
Kernel Version: 6.18.5-200.fc43.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 8 × Intel® Core™ i7-8559U CPU @ 2.70GHz
Memory: 16 GiB of RAM (15.5 GiB usable)
Graphics Processor: Intel® Iris® Plus Graphics 655
Manufacturer: Intel(R) Client Systems
Product Name: NUC8i7BEH
System Version: J72992-303

ADDITIONAL INFORMATION
This seems related to Bug 486401, because both are about how Wayland events are delivered or processed differently than X11. However this bug applies to resizing rather than scrolling, and makes the app unusable rather than just slow.