If I had to guess: synthesized mouse move events may not undergo all the same coordinate transformations as native mouse move events, and so they might cause different (wrong) coordinates to arrive to AccessibleCaret.
Bug 1580669 Comment 9 Edit History
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If I had to guess: synthesized mouse move events may not undergo all the same coordinate transformations as native mouse move events, and so they might cause different (wrong) coordinates to arrive at AccessibleCaret.