editing/selection/drag-in-iframe is a pixel test of the painting of the drag caret inside iframes. The currently checked in pixel results are wrong (not showing a drag caret), since the test has been invalid at least ever since DRT was changed to enqueue events sent between mousedown and mouseup. Moreover, the test "fails" intermittently (see URL) because it uses a timeout to predict when the iframe has loaded. Patch forthcoming.
Created attachment 13936 [details] Repair the test
Comment on attachment 13936 [details] Repair the test r=me
r20691