Improve source editing textarea field size and scrolling behaviour

[dawidurbanski]

Suggested merge commit message (convention)

Fix (source-editing): Improve source editing textarea field size and scrolling behaviour. Closes #10422.

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Additional information

Testing should be done with at least these steps:

1. Run yarn manual --files=source-editing and test source editing with default content.
2. Copy all the content from https://ckeditor.com/docs/ckeditor5/latest/features/source-editing.html to the manual test and check source editing again.
3. Build docs locally (yarn docs && yarn docs:serve) and check http://127.0.0.1:8080/ckeditor5/29.2.0/features/source-editing.html

In all cases there should be:

• No horizontal scroll.
• All the content in text area should be reachable by scrolling down.
• In manual test: textarea should be long and the body should be scrollable.
• In docs: textarea should have max height of 700px and the textarea itself should be scrollable.

[Reinmar]

For some reason, this breaks the manual test.

If you open it on master, you can see the bottom of the editor:

On your branch, you cannot scroll further than this:

And I even tried adding padding to body. It didn't help.

The problem is probably related to some unnecessary padding in the textarea:

[dawidurbanski]

@Reinmar I fixed what you have raised, and by the way simplified this ugly width calc thing. Turned out that after getting rid of css grid it was enough to apply box-sizing: border-box; onto textarea and it works just fine.

[Reinmar]

Out of curiosity, is this bug that these areas are not 100% width/height of available space resolved on your PR:

[dawidurbanski]

Yes: