1378109 - Scrollbar briefly appears on Web Developer menu show transition

Status: RESOLVED DUPLICATE of bug 940733

(Firefox :: Theme, defect)

Description

[Nicolas Chevobbe [:nchevobbe]]

Attachment: devtools_submenu_scrollbar_flash.mp4

On OSX 10.12.5, Nightly 56.0a1 (2017-07-04) 

Steps to reproduce: 

1. Click the hamburger menu
2. Click on the web developer entry


Expected result:

Everything is smooth and nice


Actual result:

At normal speed, there seems to be a small glitch during the transition.
Thus I recorded the animation and played it frame by frame (see file attached, which is a record of that). At some point we can see that a scrollbar appear on the new list, and then disappear, which explain the perceived "glitch" at full speed.
This is quite subtle due to OSX toolbar being discrete, but it feels a bit unpolished.
I guess it is possible to only show the scrollbar if it is needed at the end of the transition.