There's a few related issues which contribute to the breakage in nested writing flow:

• A block's selection occurs in mousedown, whereas DefaultBlockAppender responds to click, meaning when clicking the default block appender in a columns block, the "Columns" block will become selected before the insertion, and per the logic introduced here, avoid focusing anything since at that point, the default block appender (read only) is still present in the DOM
  • Fix may be to either change DefaultBlockAppender to append onMouseDown, or change BlockListBlock's to select onClick
• When splitting blocks in a nested paragraph, the WritingFlow logic for finding the selected block produces a null value. Presumably the newly split block is not yet in the DOM. This doesn't occur at the top-level (correctly finds the newly split block), and I'm not yet sure why there's a distinction.

Or we could add a setTimeout to the WritingFlow's block selection focusing 😬

A block's selection occurs in mousedown, whereas DefaultBlockAppender responds to click, meaning when clicking the default block appender in a columns block, the "Columns" block will become selected before the insertion, and per the logic introduced here, avoid focusing anything since at that point, the default block appender (read only) is still present in the DOM

On second thought, even though this is true, the later insertion of the default block should change the selection and trigger another WritingFlow component update...

In fact, I think these are the same issue: That the second component updates, but the new default block isn't yet present in the DOM, same as noted for splitting.

@aduth This might have an impact here #5100 It could fix some of the undesired focus calls.

Closed in favor of #5102