Prevent focus loss when selecting around the end of the last text block

[jasmussen]

We allow a user to hold shift and select text inside a paragraph block. When the user starts to select text around the end of a paragraph, the down and right arrows can cause a focus loss (focus is set on body).

Use case 1, pressing shift down in the last text block:

• Have only a single paragraph block on the page and set the caret on the last line
• Hold shift and press down to select downwards, and note how focus is lost

Use case 2, pressing shift rigth in the last text block:

• Have only a single paragraph block on the page and set the caret on the last line
• Hold shift and select text leftwards. Now press the right arrow because you want to select less text, but notice instead how the focus is lost.

In both cases, cross-block keyboard selection ideally shouldn't fire, neither when there's just one block on the page, nor when that block is the last block and you're selecting downwards.

[jasmussen]

Somewhat related, #3745 (comment):

Multi-selection shift expanding behavior differs from master, where shift-selecting upwards multiple times and then shift-selecting downward will reverse the selection direction from the originating block.

[bph]

I encountered a similar issue with Safari - putting the cursor at the end of the block and then use backspace to remove characters.
It removes not the next character but the second last character. And when I tried to type from the position, the cursor would not move .
http://recordit.co/RnP1d6E2MS

[youknowriad]

Unless, I'm missing something this is fixed.