"START_OF_SELECTED_AREA" character appears after undo

[hypest]

Describe the bug
Undoing a paragraph merge leaves behind a copy of the START_OF_SELECTED_AREA (0x0086) char.

To reproduce
Steps to reproduce the behavior:

1. Start off with this html:

<p>Word</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Press</p>
<!-- /wp:paragraph -->

2. Merge the “Press” block to the one above it
3. Undo
4. Inspect the html via the browser dev tools and verify that the special char is there before the Press word in the second block

Expected behavior
No copies of the special char should be left in the text/markup.

Screenshots

Desktop (please complete the following information):

• OS: macOS 10.14.4
• Browser Firefox 66.0.5
• Version on hash eea04cf

[aduth]

It was mentioned that this may have been fixed by #15599 . Needs additional testing or confirmation from @ellatrix .

An underlying cause is the mutation of the original block object in the logic of block-editor's effects.js, which has the negative impact of unintentionally mangling Undo history.

There is some direct mutation which still occurs in this file, which is concerning to me:

gutenberg/packages/block-editor/src/store/effects.js

Line 102 in 34d24ed

selectedBlock.attributes[ attributeKey ] = toHTMLString( {

gutenberg/packages/block-editor/src/store/effects.js

Line 137 in 34d24ed

selectedBlock.attributes[ attributeKey ] = html;

[ellatrix]

@aduth Yes, it was a bit more challenging to not mutate. I just wanted to add a line in the other PR so it's fixed.

[ellatrix]

This is fixed.