STR: 1. Visit https://jsfiddle.net/ 2. Click in the "HTML" fancy-canvasy-textbox thing (the upper left one) 3. Type a few characters (e.g. "a", and then "b", and then "c") ACTUAL RESULTS: After you type a character, an extra "ghost" version of it shows up as well, offset a bit. After you stop typing for a second or two, the ghost versions disappear. EXPECTED RESULTS: No ghost versions. I'm pretty sure this is a somewhat recent regression. I'm using Nightly 50.0a1 (2016-06-15) with a fresh profile.
(Also, the blinky caret line gets duplicated towards the upper left.)
Created attachment 8763131 [details] screencast of bug Here's a screencast of the bug, with latest Firefox nightly.
Range from mozregression: https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=cc9051869d1a9078a821da5d1639299f9e836e0e&tochange=1ee604ae873ffae665e745b3553fb1c406dbdd6a --> caused by Bug 1265237 Which helped me discover bug 1277251, which this is a dupe of (and which itself is a dupe of a more generic bug). (Though, bug 1277251 is tagged as e10s-specific, whereas I can reproduce versions of this bug in e10s as well as non-e10s profiles. I'll add a note about that on the other bug.)