"Window" input mode requires the use of core pointer to select the window to work in, and documentation of this fact
Submitted by def..@..hoo.de
When operating in "window" mode, GIMP will only receive tablet events if the cursor starts its motion inside the GIMP window. This makes "Window" mode unusable for tablets unless GIMP is fullscreened.
I know that this is an underlying problem with Xinput/GTK/whatever. That's not the issue. The issue that GIMP advertises a level of support that for whatever reason it does not have. The "window" mode should either be fixed (for instance by grabbing the tablet device) or removed entirely.