This bug appeared after upgrading to plasma 5.8, it seems to be related to changes in kscreen/multihead support. I have left screen on DVI-D-0 (labeled as no. 2 in wacom kcm) and right screen on DVI-I-1 (primary output, labeled as no. 1 in wacom kcm). In wacom kcm, I have tablet mapped to right screen (no. 1). However, after login, tablet is ends up being mapped to the left screen but with geometry of the right screen. If I restart kded5, tablet is remapped to the correct screen. Also if I disable one of the screens, mapping is not updated to reflect that (tablet ends up being mapped to roughly half of the only screen), but this could be a different bug.
So I poked around a bit and it seems to be caused by QApplication::desktop()->screenGeometry() returning invalid, pre-setup geometry for some reason i.e. in XinputAdaptor::mapTabletToScreen initially screenList contains: 1920x1080+0+0 1680x1050+1920+0 which is my "boot" geometry and is incorrect After I restart kded5 screenList contains: 1920x1080+1680+0 1680x1050+0+0 which is my actual geometry, set through KDE screen settings and applied when I log in This messes up coordinate transformation matrix and breaks screen mapping as a result Not sure if it's a Qt bug. I'm using Qt 5.7.1
It actually seems to be fixed in current releng3.0 head, but there's no release with this fix (3.0.0 tag was almost two years ago) https://cgit.kde.org/wacomtablet.git/commit/?h=releng3.0&id=72efb5c0f38bb8fc5b6136b5437a8e8a1f372067