In NWE with syntaxhighlight enabled, the lineheight of the syntaxhighlight and the text in the background differs a little bit.
I can see this on all long pages, both in Firefox and Vivaldi in de.wikipedia and Vector skin.
Note how the wavy underlines move up further down the text.
I can't reproduce this issue on en.wikipedia. I'm not aware of any personal customization that influences the lineheight, but due to T185303, I can't test that.
I can fix this by setting
.mw-body-content { line-height: 1.5; }
(default is 1.6), which is inherited by
<div class="CodeMirror-code" role="presentation" style="">, while
<pre class=" CodeMirror-line " role="presentation"> has an explicit line-height of 1.5em