FIrefox tabs are not properly clickable from top of screen

Distribution

22.1

Package version

6.4.8

Graphics hardware in use

Intel Corporation CoffeeLake-H GT2 [UHD Graphics 630] and NVIDIA Corporation GP107M [GeForce GTX 1050 Mobile]

Frequency

Always

Bug description

I would like to thank you in advance for maintaining this open source software, I really appreciate it.

As I want to be able to click the very top of a tab in order to switch to that tab in Firefox, I have disabled the title bar from within Firefox. My expectation is that I can then click at the top of a tab to switch to it, so that I don't have to focus on where to click. However when I do so, the behavior instead is that at some horizontal pixels I can click to switch to the tab, but on other horizontal pixels I cannot. Every other pixel or so, this behavior changes, so there is no way to click the top of the tab without focusing on where specifically I need to click. If I hover over the spot where I can click on the tab, the tab becomes highlighted as expected. When I hover over the spot where I (for whatever reason) can't click on the tab, the tab does not become highlighted.

Some extra information from debugging the issue:

• It occurs with Firefox CSD (title bar disabled), both when maximized AND when manually resized to the top of the screen.
• It also occurs with a fresh Firefox profile and safe mode
• It's not resolved by changing "Applications" or "Desktop" themes, nor by disabling Window Tiling or Desktop Effects.
• It's not resolved by changing the underlying Muffin WM theme (e.g., from 'Mint-Y' to 'Adwaita').

Steps to reproduce

1. Disable Firefox title bar from within Firefox
2. Move the cursor to the top of the screen where the tabs are, and slowly move left or right. You will see that every other pixel or so, the tab highlight keeps going active and inactive.

Expected behavior

The tabs are clickable at all horizontal pixels at the top of the screen.

Additional information

No response

No source files, tests, or entry points are named. Reproduce with Firefox's title bar disabled at the top screen edge, then trace Cinnamon's window and pointer handling for the alternating hover behavior; done means every horizontal pixel over the tab is clickable and highlights consistently.