Created attachment 2315 [details]
Patch which corrects issue

This is related to bug 2842, which was resolved by discarding button events which are not within the window content frame. With the current mercurial 2.0 code, on all versions of Mac OS X 10.7 through 10.11, when your cursor is at the very top of the window (easy to do in fullscreen mode), mouse button down events are not registered.

https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Miscellaneous/Foundation_Functions/#//apple_ref/c/func/NSPointInRect states "Point-in-rectangle functions generally assume that the 'upper' and 'left' edges of a rectangle are inside the rectangle boundaries, while the 'lower' and 'right' edges are outside the boundaries." Since the origin is at the bottom left, this may explain why the NSPointInRect test is failing when the cursor is at the top edge of the screen (I'm not sure why the same issue is not seen on the right edge of the screen, however).

Adding one to the width and height of the rectangle seems to correct the issue, and allows clicks at the top edge of the window to be detected. See attached patch.