On master: As admin go to the participants page for a course. Click the enrol users button to open the enrol users popup. Verify there is no visual glitch after the second auto-complete element (see screenshot). Close the popup. Repeat this test at least 10 times (more if you are generous).   On stables - test the tags element on a module settings form looks and works the same before/after this patch. In this case I don't think it's worth a big effort to make a fake test on stables. It's highly dependent on the time to render the thing (I couldn't reproduce it with the tags element for example).