theme editor doesn't seem to allow placing code below a certain line

[m-rio]

I'm trying to edit/place code beyond the certain line in the textarea but I can't. the screnshot attached shows a blue line that appears when trying to place the cursor beyond a certain point. The content seen on the screen was pasted. The off selection that appears in screenshot (does't select the exact content seen overlaying the selection) is another glitch that appears when trying to place more lines of code that accepted:

[ashelkovnykov]

Is this still occurring after this comment?

[m-rio]

@ashelkovnykov I don't think these are related because I could make changes to the content above the think blue line. I could also go to a post (published or not) modify it and update - while I don't hit the session.js issue I reported on another issue.