use transparent border for last-child row-separators
Currently, when filtering lists where rows have bottom borders we have a situation where the list shrinks and the CSS is applied after a short delay (while GTK computes in the background).
That mean you can have a listbox w/ rows, then it shrinks due to fewer rows, then the border is removed for
:last-child which causes it to shrink 1 more pixel.
To work around this in Text Editor we just set the last row to have a transparent border instead. https://gitlab.gnome.org/GNOME/gnome-text-editor/-/blob/33afcb3d0459e5f5cd76e9a11d95eb254a48f512/src/TextEditor.css#L151
It can result in the final row technically having 1 more interior pixel than the others, but the avoidance of the animation jank seems rather worth it.