Left mouse button releases too soon.

I usually play with the bow or sword. When I'm holding down the left mouse button to charge for an attack, the button seems to release too soon, even though I'm still holding it down. I've used multiple mice but the problem persists with each one. I've used a javascript event tester to see if maybe the browser thinks the mouse button is releasing, but it always shows the button is being held down.

This happens in both Chrome and Firefox, in both Ubuntu and Windows 10.

The bow is mostly useless when the mouse button releases, so this is pretty frustrating. If I've got something setup wrong or I'm not understanding something correctly please let me know.