ID:1917316
 
Resolved
Mouse screen_loc coordinates were incorrect.
BYOND Version:508.1295
Operating System:Windows 10 Pro 64-bit
Web Browser:Chrome 42.0.2311.135
Applies to:Webclient
Status: Resolved (508.1296)

This issue has been resolved.
Basically, the screen-loc given by the mouse events has an inverted y-coordinate. That is, the top gives a lower value than the bottom. It makes sense that this might be an issue considering HTML/JavaScript coordinates behave that way, haha.
Lummox JR resolved issue with message:
Mouse screen_loc coordinates were incorrect.