Summary: code blocks cause text input to be broken when lines are split

how to reproduce:

1. add a code block and some text in it
2. remove the start delimiter (```)

expected: text after delimiter gets reset to plain text

actual: see recording

This also causes other issues, such as lines being split improperly when the cursor is in the middle of the line when the user presses enter (also in recording).

Browser: Firefox on MacOS, Arc on MacOS