Reset selection after undo/redo

[ellatrix]

Description

This is a WIP!

I'm trying to solve #12002 and #12327, but it touches some things that I'm not so sure about.

The goals is to include selection state in history record so that it can be restored. The basic idea seems to work more or less, but the expectations around restoring selection after undo seem a bit different than what you would think: the very last selection change should be restored, not the selection of the last snapshot.

I try to explain.

1. You open a post => initial present, say [ { value: '' } ].
2. You place the caret somewhere => even though there is no content change, present is expected to update with the new selection, say [ { value: '', selection: 0 } ].
3. You insert some text => the present is updated again, no history is added, say { past: [ { value: '', selection: 0 } ], present: [ { value: 'test', selection: 4 } ] }
4. A second passes => the last changes are marked as persistent, say { past: [ { value: '', selection: 0 }, { value: 'test', selection: 4 } ], present: [ { value: 'test', selection: 4 } ] }
5. You undo the last change => you expect the caret to be right where it was before typing, say [ { value: '', selection: 0 } ].

This probably means that selection should be continuously passed up to the editor module (even if no content changes). There, simple selection change should not create an undo level, of course. But a content change right after a simple selection change should (but not a content change after content change).

This would be similar to how it is done in TinyMCE: the bookmark is stored before a change is made:

https://github.com/tinymce/tinymce/blob/1b4c8329a55788e3b5201b1b4df4539cb3397b7d/src/core/main/ts/api/UndoManager.ts#L196-L206

How has this been tested?

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[ellatrix]

Superseded by #16428.