Sticky panel's left position does not consider scrollable ancestors' horizontal scroll

[oleq]

📝 Provide detailed reproduction steps (if any)\

A follow-up of #5465.

To reproduce:

1. Use http://fake.ckeditor.com:8125/ckeditor5-editor-classic/tests/manual/stickypanel.html
2. Make the inner ancestor very wide (2000px)
3. And scroll it horizontally in its parent (outer ancestor).

✔️ Expected result

The sticky panel does not change its left position when getting sticky.

❌ Actual result

It gets moved a few dozen pixels away.

❓ Possible solution

This happens because we set the panel's margin-left based on the window's horizontal scroll only. We need to consider additive horizontal scroll positions of all ancestors instead

ckeditor5/packages/ckeditor5-ui/src/panel/sticky/stickypanelview.ts

Line 356 in 4f9ce3c

this._marginLeft = toPx( -global.window.scrollX );

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