Can't drag to resize window full screen horizontally

[eternalhack]

When I try to resize a window to fill the screen horizontally often times it stops a little bit short of fullscreen width. I keep dragging but it is just stuck at that point. Sometimes it works and sometimes I get the above behavior. Very frustrating, what gives?

 

[xraydoc]

When I try to resize a window to fill the screen horizontally often times it stops a little bit short of fullscreen width. I keep dragging but it is just stuck at that point. Sometimes it works and sometimes I get the above behavior. Very frustrating, what gives?

What app?

 

[eternalhack]

What app?

Safari.