Flashing vertical lines between window list items when using background blur.

Distribution

Linux Mint 22.1 Xia

Package version

6.4.6

Graphics hardware in use

Intel CometLake-U GT2 [UHD Graphics] / NVIDIA GP108M [GeForce MX250]

Frequency

Always

Bug description

This was happening on Linux Mint 22 Wilma (Cinnamon 6.2.9) as well. When I add a background blur to the panel strange vertical lines flash when I hover over window list items and are sometimes present even when I'm not interacting with the panel at all. The first screenshot is unedited and the second one has the vertical lines clearly highlighted (because they're a lot harder to see in a still image). The screenshots are a bit saturated because of #12637 but that's an unrelated concern.

Steps to reproduce

1. Select the Mint-L-Dark-Aqua theme (or any other theme, this isn't unique to that one).
2. Copy the theme's folder from /usr/share/themes to /home/user/.themes.
3. Append these lines to the end of the cinnamon.css file:

.panel-top, .panel-bottom, .panel-left, .panel-right {
  background-color: rgba(47, 47, 47, 0.3);
  background-blur: 2px; }

4. Restart Cinnamon (CTRL + ALT + ESC).
5. Hover over the window list items with your mouse.

Expected behavior

There should be no flashing vertical lines, they should either always exist (as an intended border), exist only on hover, or never exist.

Additional information

I can reproduce this with any theme I try, I've tried several built-in themes as well as a few third-party themes as well. I have tried explicitly setting no borders (border: none;) for grouped window list items but that doesn't appear to work.

Reproduce the issue with the copied theme's cinnamon.css and the added panel background-blur rules, then restart Cinnamon and hover over window list items. Compare the result across the mentioned themes and hardware setup; done means the vertical lines no longer flash or appear unexpectedly.