Pages with an SVG “jump” in Code Editor so I can't edit the code

This is an unusual one.
I'm trying to edit a page using code editor and have found a really strange bug/issue. Perhaps unique to me, I don't know. Using Wordpress 5.7.

I am editing (or trying to anyway) a page using the code editor. For some reason, in code editor, the cursor "jumps" to the bottom of the page, and keeps jumping back there multiple times a second. So I can't edit anything that's not the bottom line.

Testing this a little, I noticed I can edit the code on pages that don't have an SVG, but any pages with an SVG have this strange behaviour.

I wish I could film the code, so you can see it move. In the SVG part of the code, every second, the following code appears, then disappears. This code occurs after <svg and before xmlns="http....":

aria-hidden ="true" height="1em" width = "1em"

That code literally flashes in and out, with everything after it moving in, then moving out.

Anyone else seen this behaviour and know what causes it? Also how to resolve it?

To repeat, pages without an SVG behave as normal - i.e. I can edit the code. It's only pages with an SVG that display this odd behaviour.