STR: 0. Have the Gecko Profiler installed ( https://perf-html.io/ ) 1. Load https://bugzilla.mozilla.org/attachment.cgi?id=9020942 2. Start profiling. 3. Focus the text and type a few characters (e.g. "abc") 4. Capture profile. 5. Look for reflows on the Marker Chart (or as blue bars in the top timeline view) ACTUAL RESULTS: My profile: https://perfht.ml/2SSKPJH There's a >20ms reflow with each keypress. EXPECTED RESULTS: We shouldn't have that much work to do -- the reflows should be < 1ms. (I'm spinning this off of bug 1376536 -- this testcase was initially attached as a reduced testcase there, but it turns out that the patch that'll be fixing that bug doesn't actually fix this testcase.)