Will attach screenshot
This is probably a rounding issue, the line height is probably not an integer number of pixels but we round selection rects to pixels for speed. One way to fix this would be to eliminate (some) gaps by making text selection extend vertically to the top and bottom of the line bounds.
Created attachment 296034 [details] screenshot on PMO Hmm, I've seen this on planet.mozilla.org text for a long time. I always assumed it was a feature, sort of like lines on ruled paper!
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