This particular STR works in my case about 70% of the time, but I guess other similar STRs are also possible:

1. Latest Nightly, using Windows 10 on a password-protected laptop (this last bit probably doesn't matter, putting it here just because I've seen similar bugs filed in the past which were closed due to being unable to reproduce, so the more details the better, I guess)
2. Overflow the tab strip so that the All Tabs button would appear, and overflow the height of the screen when pressed
3. Click the All Tabs button -- the panel is overflowed, but the bottom part of the panel is visible -- all good so far
4. Close the lid of the laptop, let it go to sleep
5. Open the lid and quickly input the password to log back in
6. This part I think is the most important- Make sure that the taskbar flickers a bit after you log in. Most (if not all) of the time I couldn't reproduce was (presumably) because it didn't flicker. Continue to the next step but if the issue doesn't surface, repeat steps 4-7
7. Click the All Tabs button -- the bottom part of the panel is beneath the taskbar, making the buttons/items there unclickable until you restart Nightly

Some notes:

1. Restarting explorer.exe doesn't help.
2. In case my setup configuration is important: I'm using a 13" laptop, Windows 10 v1809, the laptop monitor (on which the STR above was tested) DPI is set to 150%, the other 2 connected 22" monitors are set to 100% DPI, all monitors are at 1920x1080 resolution.
3. Presumably this STR won't work on high-end laptops, because there's little chance that the taskbar will flicker on step 6. So (again, presumably) the 2018 Reference Laptop should be suitable to test this STR. But fwiw, I think I've seen this once or so on my high-end PC.

Unfortunately I can't check for the regression range (using moz-regression, at least) on the environment this laptop is in due to network restrictions.

What causes the taskbar flicker?

(In reply to :Gijs (he/him) from comment #2)

What causes the taskbar flicker?

Yeah, I'd like to know the answer to that myself...
Not sure tbh. I think this is due to Windows trying to sync it between all the connected monitors?
It's a baseless hypothesis though. I'll test this tomorrow and/or try to test this some more on my PC.

Okay, some new observations:

1. I couldn't reproduce this on my laptop when it is disconnected from the docking station (hence disconnected also from the other 2 monitors).
2. Simply connecting or disconnecting the laptop from/to the docking station made this issue appear. To reduce variables here, I connected one of the monitors straight to the laptop (with the laptop disconnected from the docking station) and I could still reproduce this afterwards.
   2a. When doing so, I noticed that the entire content area (or possibly the chrome UI) shifted down below the taskbar (this is also when the "flicker" occurs), and then back up to normal state (above the taskbar) a few moments after. It seems that some of the Firefox components are not aware of the new height of the browser?

A (more) reduced STR for me is:

1. Latest Nightly, Windows 10. (Not sure if required: use a somewhat slow PC)
2. Overflow the tab strip so that the All Tabs button would appear, and overflow the height of the screen when pressed
3. Click the All Tabs button -- the panel is overflowed, but the bottom part of the panel is visible -- all good so far
4. Connect/disconnect an external monitor and wait for Windows to to its thing and sync everything up -- this is when the taskbar "flicker" should occur
5. Click the All Tabs button -- the bottom part of the panel is beneath the taskbar, making the buttons/items there unclickable until you restart Nightly

(In reply to :Gijs (he/him) from comment #2)

What causes the taskbar flicker?

I think it's caused by connecting/disconnecting an external monitor.
I assume that this happened to me initially because the monitors are disconnected when going to sleep, and upon waking up they are being reconnected.

I don't have a setup to easily test on Windows with an external screen, but this smells like a widget bug around what it claims the availHeight/availTop of the screen in question is.

I managed to reproduce this on my PC which has only one monitor connected. Now sure what were the steps but I think it was after Windows woke up of sleep.

The priority flag is not set for this bug.
:jimm, could you have a look please?

For more information, please visit auto_nag documentation.

Are you still able to reproduce this?

This was fixed by bug 1746774.