Triple click incompatible with nested blocks

[caifara]

Do you want to request a feature or report a bug?

bug

What's the current behavior?

Upon using triple-click + backspace to delete the first of two adjacent custom blocks A and B (A itself containing a paragraph), the second block (block B) becomes nested in block A.

These tickets/PR's are related: #1605, #950, #863 (comment)

The gif start with a triple-click + delete having an expected result (everything else stays the same) and continues with two unexpected ones (the adjacent block becomes nested in the first block).

https://jsfiddle.net/caifara/62e3bxgL/5/

Mac OS 10.14, Chrome 71.0.3578.98, Slate 0.44.9, Slate-react 0.21.15

What's the expected behavior?

The selected line gets deleted, everything else stays the same.

[ianstormtaylor]

Fixed by #3093.