Oh, you just have to ask

I have a solution for the Shift+C bug.
I think it's a leftover from the old keyboard Kindle models, where Shift was used as a modifier key, so Shift-C was mapped to Copy, Shift-F to find...Anyway, Lab126 borked the GTK-IM library and deleted the fallback solutions.
The solution is to get the events directly from the system, bypassing the GTK-IM library.
See
this commit that solves the issue for the GTK version of Textadept.