https://bugs.kde.org/show_bug.cgi?id=510513
Bug ID: 510513
Summary: applications cannot receive more than simultaneous
touch events
Classification: Plasma
Product: kwin
Version First unspecified
Reported In:
Platform: Fedora RPMs
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: Gestures
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
Created attachment 185707
--> https://bugs.kde.org/attachment.cgi?id=185707&action=edit
screenshot of virtual midi controller application
I created a virtual midi controller that is designed to take full advantage of
10-point multitouch touch screens, such that the operator may play chords with
up to ten fingers using a standard pair of human hands.
However, when KDE receives more than two touch events in a short interval, KDE
sends two touch events, and what I assume are two touch cancel events (SDL2
does not have touch cancel events, so it gets unknown events, but they have the
same event number as SDL3's touch cancel events). This effectively renders it
impossible to play more than two notes in a single chord, but also it means the
two notes that did play get stuck because I haven't figured out how to handle
the cancel events yet.
I am told that this is a hardcoded behavior in KWIN, and there is no way to
disable this behavior. Disabling the effects of this behavior (having only one
virtual desktop, and disabling that one desktop effect) does not help the
situation: the additional touch events are always withheld from the
application.
STEPS TO REPRODUCE
1. simultaneously press three to ten fingers onto a multitouch touchscreen
OBSERVED RESULT
observe that KDE never propagates them to an application under any
circumstances
EXPECTED RESULT IN A PERFECT WORLD
Application developer would be able to communicate to KWIN by some means that
their program should receive all touch events when it has input focus.
EXPECTED RESULTS IN A FLAWED WORLD
Operator would be able to disable KWIN gestures in such a way that applications
always receive all touch events instead of them being retroactively cancelled
to two.
SOFTWARE/OS VERSIONS
Operating System: Fedora Linux 42
KDE Plasma Version: 6.4.5
KDE Frameworks Version: 6.18.0
Qt Version: 6.9.2
Kernel Version: 6.16.9-200.fc42.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 20 × 13th Gen Intel® Core™ i7-1370P
Memory: 64 GiB of RAM (60.5 GiB usable)
Graphics Processor: Intel® Iris® Xe Graphics
Manufacturer: Framework
Product Name: Laptop (13th Gen Intel Core)
System Version: A7
ADDITIONAL INFORMATION
My application can be found here:
https://github.com/Aeva/mollytime/tree/excelsior/false_start (run pads.py after
installing pygame and alsa-midi into a python virtualenv via pip.
Alternatively, https://code.benco.io/touchmidi/layouts/generic-keys.html is a
similar multitouch instrument written by someone else. It requires a web
browser that supports web-midi, such as chromium.
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