I can reproduce it with few apps (mousepad, firefox, claws-mail). For example with firefox when I scroll fast through a webpage, it suddenly stops scrolling and doesn't respond to mouse events until I press any key on my keyboard, it then processes all delayed mouse events along with key press. When I comment out #define HAVE_XI2 1 in config.h everything behaves normally. Bisected down to: https://git.xfce.org/xfce/xfwm4/commit/?id=6013f1ee9b3831991db6a0fb896f20537532b6ce
Maybe related is bug #14695. Can you try the proposed patch if it fixes this too?
I can confirm that patch in #14695 fixes problem for me.
*** This bug has been marked as a duplicate of bug 14695 ***