Merging blocks after split causes crash

[mkevins]

Description

The editor crashes when merging blocks after splitting. This is very similar and may be a partial regression of #921, which was fixed by #949.

This seems to be fixed by #1032, however, it is difficult to follow why this is the case.

In the diff, isSelected can be set from true to false (but not false to true) based on the conditions here: https://github.com/WordPress/gutenberg/pull/15833/files#diff-4828a21853e899e5a36faecfa96d83e8L926. I suspect setting isSelected to false alters the control flow in other areas of the code, resulting in an invalid state, but I have not invested enough time to explore that.

Steps to reproduce

1. Place caret in paragraph block.
2. Press [Enter] to split block.
3. Tap the beginning of the second split block to place the caret there.
4. Press [Backspace] to merge the blocks.

Expected

The blocks should merge into one.

Actual

The editor crashes 💥

Screenshot

[mkevins]

Closing this, as it has been resolved by #1032.