Issue Overview

So Gutenberg's history is managed by changing redux state. This works really well if the undo and redo buttons are fired by the keyboard shortcuts (e.g. Ctrl + Z), where moving through the page history can shift the focus to the corresponding block correctly.

However, if the user has navigated to the toolbar, and pressed Space / Enter to active the undo and redo buttons, the focus becomes very inconsistent

1. It can stay on the button itself. This happens if the undo puts you in the PostTitle. This is likely because the PostTitle focus state is not encapsulated in the redux state.
2. It can jump to the block where the last history change was saved. This is likely because the block focus is often encapsulated in the redux state.

Note, with (1), if the undo button becomes disabled because you get back to the start of the undo history, the focus is lost, and the focus returns to the body, breaking keyboard navigation.

With (2), this prevents a user from making multiple undo operations in a row. You could rather easily argue that most people would be using the Ctrl + Z shortcut, though (note, however, that this is a different operation from clicking / keyboard activating that button though ... as it goes through TinyMCE). The behaviour just seems to be inconsistent. We need to decide what should happen.

a) Should keyboard activating the undo / redo buttons always transfer focus back to whatever block previously had focus? What if something else had focus in the block?
b) Should keyboard activating the undo / redo buttons always keep focus on the button. Would it work differently then if you clicked on it with the mouse?

Steps to Reproduce (for bugs)

Situation 1

1. Create a new post
2. Focus the post title
3. Type "One word"
4. Press Alt + F10 (or equivalent)
5. Navigate with the arrow keys to undo
6. Press Space to trigger Undo repeatedly until the button gets disabled

Expected Behavior

The focus stays somewhere in the toolbar, or it jumps straight to the PostTitle immediately.

Current Behavior

The focus stays in the toolbar until the last undo, and then focus is just lost.

Situation 2

1. Create a new post
2. Focus the "Write Your Story" block
3. Type "One two three four five"
4. Press Alt + F10 (or equivalent)
5. Navigate with the arrow keys to undo
6. Press Space to trigger Undo

Expected Behavior

The focus stays somewhere in the toolbar, or it jumps to the last editable focused in the history.

Note: I also thought that pressing that button would trigger an undo in TinyMCE instead, but it doesn't. This means that Ctrl + Z and activating that button have completely different meanings. Is there a better way we can handle that?

Current Behavior

The focus jumps to the last editable focused in the history, so this can be fine as long as it is consistent.