The impression I get from playing with the interface is that at some moment the browser decides to "wrap" some of the components of the page in the source code view, that leads to the unwanted behaviour.
When we'll come to this, it might be interesting to compare with the behaviour of similar web front ends (GitHub, BitBucket, GitLab) that seem to handle this corner case seamlessly.

I found the cause of the observed issue in T2456. This was effectively a CSS issue: mixing float and flex rules can lead to this kind of weird layout on high resolution screens.

Nevertheless, font size consistency must still be achieved so keeping that task open.