Codeblock adds additional characters

[Valiantiam]

When you delete a character on a codeblock fresh line, it adds that character to the end of the line repeatedly for each additional character you type.

To Reproduce
Steps to reproduce the behavior:

1. Make a codeblock
2. Type a single character on its own line.
3. Backspace and replace it with a different letter.
4. It now starts putting copies of the original letter on that line for every character you type.

Expected behavior
It should not add additional characters.

Screenshots

Desktop (please complete the following information):

• OS: [W10]
• Browser [Chrome]
• Version [90.0.4430.93 x64]

[tommoor]

wild, It's something to do with the decorations introduced by the Prism plugin – disabling highlighting on the block causes the bug not to reproduce.