Screen edge compensation causes misalignment in shader

[webstrand]

When using a shader to zoom in on the selection area, the selection rectangle is noticeably offset from the cross-hair by 1px.

The issue seems to be caused by the compensation for the edges of the screen, specifically adding glm::vec2(1*(!lx), 1*(!ly)) to the mouse's position:

slop/src/slopstates.cpp

Line 123 in f5e4f95

memory.rectangle->setPoints(startPoint+glm::vec2(1*lx,1*ly), mouse->getMousePos()+glm::vec2(1*(!lx), 1*(!ly)));

If I understand correctly, the compensation is necessary or it would be impossible to select the bottom-most and left-most pixels of the screen? However, I don't believe that there's a way to reverse this compensation to position the cross-hair accurately in the fragment shader (I'm a novice, though).