The top bar's height is not stable

The top bar is meant to be 51 pixels high and the left bar should be immediately below it. In FireFox for some reason it is rendered 50px high and there is a single pixel whitespace room between the two. In Chrome, depending on the zoomfactor, you see the same white line and when you inspect the top bar height at the different zoom levels, you see it switching from 51 at 100% to 50 at 110% to 50.778 at 125%, etc.