You can see this happen to the "p" on your animated gif here:
https://i0.wp.com/css-tricks.com/wp-content/uploads/2021/06/syntax-highlighted-textarea.gif
and also if you try this example:
<span></span>
<test> <-- this line overwrites the bottom of the p in "span" of the above line...
where by the descender part of the "p" is overwritten by the next line...
Proposed fix:
as the lineheight is set to 1.5 via:
code[class*="language-"], pre[class*="language-"] {
it should be also set to 1.5 on (and not 20pt) here:
#editing, #highlighting, #highlighting * {