Experiencing the same issue, thought it was my custom CSS somehow.
This is an easy fix: just apply overflow-x: scroll; to the details element for code execution blocks. Unfortunately there's no CSS class applied to select only the code blocks, but you should be able to use this CSS:
div.chat_bubble > div.content > details:open {
overflow-x: scroll
}
This will add a horizontal scroll to all open details elements in Assistant chat bubbles.
