Created attachment 9102 panel with style "solid color" Setting "Panel Preferences" -> "Appearance" -> "Background" -> "Style" to "Solid color" results in missing and/or invisible pixels above and below launchers and separators. Other plugins are displayed correctly. See the screenshot: The desktop color is red, image is cut at panel's dimensions. No red pixels should be visible, but they are.
Created attachment 9103 panel with style "None (use system style)" Adding another screenshot with style "None (use system style)". No red pixels are visible. (Though this reveals another issue: The panel row size is still set to 48 pixels, but the panel has a height of 49 pixels. That's not correct, no?)
What you describe is the panel's border, which is drawn *in addition* to the panel's size. There's a 1px border on each side that is not "attached" to a screen border, so if you have a top-aligned panel that is 100% width, you get border-bottom: 1px; I'll see if I can reproduce, curious that it would just be those two plugins...
Just noticed this depends on the selected theme. "Adwaita" is ok, "Adapta" has the border issue.
Adapta is available from Github: https://github.com/adapta-project/adapta-gtk-theme I installed the Arch Linux package: https://www.archlinux.org/packages/community/any/adapta-gtk-theme/
My best guess without deeply digging is that this is where the problems start: https://github.com/adapta-project/adapta-gtk-theme/blob/master/gtk/sass/3.24/_3rd-party.scss#L2356 Not sure why they're un-setting the borders. In any case, I don't see how this could be meaningfully prevented by the panel.