Toolbar tray items are unclickable on really small viewport (below 16.5em)

Hello @huzooka,
I have generated a new patch in accordance with D9 because your patch is in D8 and the ticket is for D9, thus the patch is not applying.
There is a better solution presented for screens less than 16.5 em since if we attempt with z-index, the other options won't be seen.
If we try with z-index then other option will not visible so provided one better solution for below 16.5 em screen
In response to a request, I've included CSS with a specific focus on breakpoints below 16.5 em to achieve narrow viewport design.
Since everything is working fine on above 16.5 em, it is a good idea to limit CSS to max-width: 16.49 em so that it won't conflict with any existing code.
I tried this patch with these admin theme Bartik, Claro, Olivero, Seven, and stark and it working fine as expected.
Note: Also this patch is working fine with with admin toolbar module.
Please review.

For ref attached screenshot.

Seems like a bug that could benefit from a test case.