If you drag the back button anywhere, the height of the toolbar shrinks. This doesn't happen in compact mode and it feels much more stable. Even though you can't remove the back button, the interaction doesn't feel great. A min-height on the toolbar should solve the problem.

Jared, is this something you could take a look at?

[Tracking Requested - why for this release]: UI regression in 57 during customize mode.

Comment on attachment 8914591 [details] Bug 1403382 - Dragging the back button out of the toolbar in customize mode shouldn't affect the toolbar height. https://reviewboard.mozilla.org/r/185938/#review190976 ::: browser/components/customizableui/CustomizeMode.jsm:1869 (Diff revision 1) > + let toolbarParent = draggedItem.closest("toolbar"); > + if (toolbarParent) { > + toolbarParent.style.removeProperty("min-height"); > + } We should probably do this in `_onDragEnd` as well, as not every drag ends with a drop? Unfortunately I don't think we can only do it there, because there are some early exits in `_applyDrop` that will not call it (maybe they should? Hard to tell...).

Pushed by jwein@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/2c2f9dc1f79b Dragging the back button out of the toolbar in customize mode shouldn't affect the toolbar height. r=Gijs

Comment on attachment 8914591 [details] Bug 1403382 - Dragging the back button out of the toolbar in customize mode shouldn't affect the toolbar height. Approval Request Comment [Feature/Bug causing the regression]: bug 1387313 [User impact if declined]: visual glitch while customizing and moving the back button around [Is this code covered by automated tests?]: no [Has the fix been verified in Nightly?]: no [Needs manual test from QE? If yes, steps to reproduce]: no [List of other uplifts needed for the feature/fix]: none [Is the change risky?]: no [Why is the change risky/not risky?]: straightforward javascript change that fixes the height of the toolbar before hiding the button and clears the fixed height after the drag has finished [String changes made/needed]: none

Comment on attachment 8914591 [details] Bug 1403382 - Dragging the back button out of the toolbar in customize mode shouldn't affect the toolbar height. Improve photon, taking it.

User-facing bug with no automation coverage and without a fix confirmation in Nightly. Flagging for manual testing.

I have reproduced this bug with Nightly 58.0a1 (2017-09-26) on Windows 8, 64-bit. The bug's fix is now verified on Latest Nightly and Latest Beta Latest Nightly : Build ID 20171006100327 User Agent Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:58.0) Gecko/20100101 Firefox/58.0 Latest Beta : Build ID 20171005195903 User Agent Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0 [bugday-20171004]

I have reproduced the issue mentioned in comment 0, using an affected Firefox 58.0a1 build (BuildId:20170926220106). I have verified that the issue is not reproducible using Firefox 57.0b6 (Build Id:20171005195903) and Firefox 58.0a1 (BuildId:20171009100134) on Windows 10 64bit, macOS 10.11.6 and Ubuntu 16.04 64bit. Thanks Anika for the help!