I don't know whether this is a problem with Krita or kxkb, but Krita is the only application that's experiencing some kind of problems, so I'm reporting here.

SUMMARY

When I resize brush using shift + drag, the resizing mode is not deactivated until I click on some menu/tool/layer.

After a lot of experimentation I found out that it's related to kxkb setting "Switching to another layout -> Both Shifts together"

STEPS TO REPRODUCE
1. Open "System Settings"
2. Go to Input Devices" -> "Keyboard"
3. In the "Layouts tab" in "Shortcuts for switching layout" group set "Both Shifts together" as the "Main shortcuts" (alternatively this can be changed in the "Advanced" tab)
4. Open Krita
5. Try to resize brush using Shift + drag

OBSERVED RESULT

The brush is kept being resized even when Shift is no longer pressed.

EXPECTED RESULT

The brush stops being resized.

SOFTWARE/OS VERSIONS

Note: this has been bugging me for a long time (at least over a year), so it's not specific to any KDE/krita version. Only now I tried to debug it by starting with a clean profile and then iteratively adding old config files until the problem reappeared.

Operating System: Arch Linux
KDE Plasma Version: 5.25.5
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.6
Kernel Version: 5.19.13-arch1-1 (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Core™ i5-6500 CPU @ 3.20GHz
Memory: 15.6 GiB of RAM
Graphics Processor: AMD Radeon RX 480 Graphics