SUMMARY I have never been able to get the "disable touchpad while typing" option to work on my laptops with Synaptics touchpads. I finally did a bit of searching and found the application called "touchpad-indicator." Installing this solved the problem. Perhaps you should take a look at the code in that app to see if you can use it to fix the problem with disabling the touchpad. STEPS TO REPRODUCE 1. Turn on "Disable touchpad while typing" in the SystemSettings applet 2. Go someplace where you need to type. 3. Watch the cursor move all over the screen. Occasionally whole blocks of text will be deleted as well. OBSERVED RESULT See #3 EXPECTED RESULT Touchpad disabled and no problems with typing SOFTWARE/OS VERSIONS Windows: macOS: Linux/KDE Plasma: Currently Kubuntu 20.04 (development) but the problem has persisted across versions. (available in About System) KDE Plasma Version: 5.18.3 KDE Frameworks Version: 5.68.0 Qt Version: 5.12.5 ADDITIONAL INFORMATION
That's because the feature basically doesn't work at all using the legavy Synaptics driver. This was one of the reasons why the author of that driver (Peter Hutterer) abandoned the Synaptics driver and wrote Libinput, where it works very well. Can you try installing the libinput driver and seeing if it works for you?
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It appears this problem is resolved in Kubuntu 20.04 which appears to be using libinput.
Thanks for the update; changing status.