Bug 452967

SUMMARY
On Plasma Mobile (which uses kwin_wayland), popup menus in any GTK application do not respond to touch input. Touching any menu entry or any other control in the GTK popover just closes the popup without taking any action.

The same effect can be seen for Chromium popups, including ones from QtWebEngine (though Angelfish ≥ 21.12 has a workaround using QML popups instead, so you will not be able to reproduce the issue with recent versions of Angelfish).

This happens only for touch input using Wayland. Plugging in a USB mouse into the PinePhone and clicking with it works. Forcing the application to use X11 (using GDK_BACKEND=x11 for GTK apps or QT_QPA_PLATFORM=xcb for QtWebEngine apps) also works (sorta, because that causes several other issues such as no virtual keyboard coming up).

This issue makes most GTK applications unusable on Plasma Mobile (e.g., Password Safe cannot be used because you cannot add a new password entry without going through a popup), and reduces the functionality of most of the remaining ones (e.g., in Geary, you cannot autocomplete addresses, switch between plain text and rich text (HTML) composing, etc.), so IMHO it should be treated as a showstopper.

According to GTK developer Carlos Garnacho, the issue is that the GTK popover/popup menu is a subsurface, and kwin_wayland is incorrectly sending the touch event to the main surface instead.

STEPS TO REPRODUCE
As stated above, make sure you are using:
1. Wayland, both in KWin AND in the application (the bug canNOT be reproduced with X11), AND
2. touch (finger) input (the bug canNOT be reproduced with a mouse).
A good device to try it on is a PinePhone running the default Manjaro ARM Plasma Mobile edition.

Once that is set, there are several ways to reproduce the bug, e.g.:
* In Geary, I "click" (touch) Inbox, then the "new message" button, then the "⋮" menu button. The popup menu that results does not work.
* Also in Geary, the autocompletion popup in the "To" field (when I start typing a known address) does not work.
* Password Safe is not usable because the "Add" popup does not work. (I do not remember the exact way to get to it because I uninstalled the application due to being unable to use it because of this bug.)
* To see that this also happens with Chromium/QtWebEngine popups, fire up Angelfish < 21.12, or presumably another QtWebEngine browser that does not have Angelfish's workaround, and try opening https://www.falter.at/lokalfuehrer and using the search filters there (the popups like "ALLE BUNDESLÄNDER").

OBSERVED RESULT
The popup closes without any action being taken.

EXPECTED RESULT
The click on the popup is processed.

SOFTWARE/OS VERSIONS
Manjaro ARM PinePhone Plasma Mobile Edition
Last tested with:
KDE Plasma Version: 5.24.2
KDE Frameworks Version: 5.92.0
Qt Version: 5.15.3 with KDE backports

ADDITIONAL INFORMATION
This longstanding issue was initially reported at:
https://invent.kde.org/plasma/plasma-mobile/-/issues/65
There, it was claimed to be a GTK issue, so:
https://gitlab.gnome.org/GNOME/gtk/-/issues/3737
was filed. Then nothing had happened for months. From a log file I uploaded, GTK developer Carlos Garnacho was able to locate the issue in KWin:
https://gitlab.gnome.org/GNOME/gtk/-/issues/3737#note_1385359
and Vlad Zahorodnii intended to investigate further, but nothing has now happened for 2 months. So I am filing this here now, in the hope that it will finally get the attention of somebody who is able to fix it.