Fix Textarea autosize causing scroll anchoring issues in Firefox

[Saracaen]

Description

This fixes the issue described in #18528 without any side effects. Upon further examination of the issue in Firefox, I noticed the following warning message in the console:

Scroll anchoring was disabled in a scroll container because of too many consecutive adjustments (10) with too little total distance (0 px average, 0 px total).

The original solution with setInitialHeight made it so that the check "this.wrapperEl.style.height === newHeight" was only true when the textarea was at its initial height. This causes issues when you have multiple lines and you are typing on one of the existing lines without growing or shrinking the textarea. In that case the "this.wrapperEl.style.height === newHeight" would be false, despite nothing having really changed.

The fix shrinks the textarea itself, determines the scroll height, and then restores the original textarea height. This gives a more accurate check on whether the height really needs to change.

Visual changes

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.

[Saracaen]

I have no idea why this one test appears to fail, but it seems unrelated to my changes.

[Saracaen]

Looks like a spurious error, considering that it passed upon retriggering the actions.