Issue Overview

With a very specific sequence of mouse movements and selections, it's possible to have the cursor active in a block, and have multiple blocks selected, resulting in unexpected behaviour.

Steps to Reproduce (for bugs)

Using Chrome 59 (exact version, 59.0.3071.109)

1. Create three text blocks, with the text "top", "middle", and "bottom".
2. In the bottom block, place the text cursor at the end of the word "bottom".
3. Drag to the beginning of the word "bottom" to select it.
4. Release the mouse button.
5. Move the mouse cursor to the end of the word "bottom".
6. Drag straight upward to select all three blocks.
7. Drag the mouse cursor to the beginning of the word "top" in the top block.
8. Drag the mouse cursor straight down to the bottom block so it is at the beginning of the word "bottom".
   You should see the word "bottom" is no longer selected, and the text cursor is at the start of the word "bottom".
9. Continue dragging upward to select all three blocks. You should see the text cursor is still active in the bottom block, but all three blocks are selected.
10. Press backspace, and the bottom block will merge into the block above it and lose focus.

Expected Behavior

I shouldn't be able to edit a block while having multiple blocks selected

Current Behavior

But, in Chrome 59, I can. (But not in 60, or Firefox)

Todos

• Tests