Comment 72 for bug 1568604

Adding this in case it helps identifying what is going on.

Here is the upper part of a screen capture. The red dot is the position of the mouse cursor at the time of the capture. Notice how the "g"s above the y-position of the cursor have a glitch, but not the one below. This happens systematically in Thunderbird and updates as the mouse cursor moves (there is a form of hysteresis : when the mouse cursor moves down, a line is affected after the cursor has passed the line below it ; when moving up, the glitch ceases for a given line only when the cursor reaches it).