I have the same issue, in 0.6.5.
Here’s the output from xev, for that particular button:
ButtonPress event, serial 31, synthetic NO, window 0x5200001,
root 0x1ea, subw 0x5200002, time 76288037, (41,43), root:(4525,63),
state 0x0, button 8, same_screen YES
EnterNotify event, serial 31, synthetic NO, window 0x5200001,
root 0x1ea, subw 0x0, time 76288037, (41,43), root:(4525,63),
mode NotifyGrab, detail NotifyInferior, same_screen YES,
focus NO, state 0
KeymapNotify event, serial 31, synthetic NO, window 0x0,
keys: 4294967274 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
ButtonRelease event, serial 31, synthetic NO, window 0x5200001,
root 0x1ea, subw 0x5200002, time 76288293, (41,43), root:(4525,63),
state 0x0, button 8, same_screen YES
LeaveNotify event, serial 31, synthetic NO, window 0x5200001,
root 0x1ea, subw 0x0, time 76288293, (41,43), root:(4525,63),
mode NotifyUngrab, detail NotifyInferior, same_screen YES,
focus NO, state 0