Can't draw snapshot border as precisely as in the past

I use the snapshot tool a lot and have always been able to draw the box precisely so that the edges of the copied material correspond exactly to the position of the crosshair. Suddenly this isn't the case -- the blue selection area only very generally encloses what I'm trying to copy, meaning I must sweat to crop it further in the target application after pasting.
 
Whaa happened? I don't believe I've done an update recently.