Custom HTML: Fix scrollbar becoming non-functional after switching tabs

[Mustafabharmal]

What?

Fixes a Custom HTML modal issue where the editor scrollbar can become non-functional after switching between HTML/CSS/JS tabs.

Closes #78545

Why?

Autosize initialization may run while the tab panel is not fully visible during tab transitions, leading to an incorrect initial size and broken scrolling behavior until another input event occurs.

How?

Use PlainText async autosize mode for the three tab editors (HTML, CSS, JS).
This defers initial autosize execution until after tab visibility updates complete.

Testing Instructions

1. Insert a Custom HTML block.
2. Open the Edit HTML modal.
3. Add enough content in HTML so vertical scrolling is needed.
4. Confirm scrollbar works in HTML.
5. Switch to CSS and JS tabs (for users with unfiltered HTML capability), then back to HTML.
6. Verify scrollbar remains functional immediately after each tab switch.
7. Verify no extra input (for example, typing a space) is required to re-enable scrolling.

Screenshots or screencast

Before:

Before.HTML.block.Fix.mov

After:

After.HTML.block.Fix.mov

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