Created attachment 146443 [details]
Test program demonstrating bug

I'm embedding a DrawingArea in a ScrolledWindow and capturing mouse events. 
When the height of the canvas approaches 0x7FFF and a window grab occurs past
the bottom of the window (hold left button down and the mouse moves below the
window) the EventMotion events report spurious positions that look like a
signed 16-bit overflow.

I've attached a program written in Vala to demonstrate this.  Scroll to the
bottom of the window, press and hold the button somewhere in the window, then
drag the mouse beyond the bottom edge.  The debug output will show the
y-coordinates going from 32767 to -32767.

Note that the EventMotion coordinates are fine when the window grab is not
active, even if the canvas has a height > 0x7FFF.  However, if a window grab
does occur at a taller height, the bug still occurs.  (A height of 0xFFFF will
overflow to 0).

To make this file, install latest Vala (I'm using Vala 0.77) and build with
this command-line:

valac --pkg=gtk+-2.0 test.vala