Classic Editor: With the Firefox 85 update, the editor toolbar is not fixed at the top of the editor.

[happychait]

Steps to reproduce the behavior

1. In the latest Firefox 85, go to WP Admin and open the classic editor.
2. Click on the editor area and scroll down.

What I expected to happen

The toolbar on the TinyMCE editor should stay fixed as it does on the earlier version of Firefox (84) as shown in this video: https://d.pr/v/6V9tDm

What actually happened

The TinyMCE toolbar doesn't stay fixed as shown in this video: https://d.pr/v/x7mSoe

Context

User report: 27492177-hc

Browser

Firefox 85

[lakellie]

Another user report in 15610477-hc

[xpurichan]

Another report in 3717620-zd.

[dolgelukkig]

Another user #22529379-hc

[vparkhere]

Another user in #3711096-zd

The issue is intermittent, sometimes it shows up and sometimes it doesn't. Here's a side by side video, both Firefox browsers, where it's working in one page but not the other. https://d.pr/v/JbdVLO

[inaikem]

Another issue in #3763993-ZD

[bsanevans]

User above (3763993-zd-woothemes) sent us some additional information.

• They experience the same issue on Safari on an iPad 2
• They DON'T experience the issue on their Samsung Galaxy, with Chrome

[fresatomica]

Reported here: https://wordpress.com/forums/topic/classic-editor-scroll-bar-for-text-missing/

[KokkieH]

https://wordpress.com/forums/topic/classic-editor-101/

[kwight]

@taggon for Fix The Flows consideration, requested in p1625067642117400-slack-C02FMH4G8