So err.... I actually found a solution
I am using a HUION H420
In Windows under Device Management > Mice and other pointing devices
Under here there are several "HID-compliant mouse" one of them was my tablet, (I found out through trial and error) Right Click > Disable.
Restarted Photoshop; then it worked fine for me.
This issue is definitely a driver issue, possible conflicts between Photoshop and drivers or possibly Windows and drivers.
TBH this is more of a work-around than a solution, but since I can't write drivers or have the sourcecode for Photoshop...
Good luck
Uhh... if there's a rule against bumping really old threads I'm sorry