Fix scroll Panel rendering for threads

[toger5]

When using rounded corners the current mx_AutoHideScrollbar css class causes issues.
If we use the browser specific scrollbar hiding flags, we can make it look better. If there are reasons not to use those, this can be closed.

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After:

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Fix scroll Panel rendering for threads (#7105). Contributed by @toger5.

Preview: https://618bb4b2cdbaed0bb8c8ea81--matrix-react-sdk.netlify.app
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[germain-gg]

Thank you for filing this, I've already worked on this as part of #7100 should be ready for review this afternoon.

That will fix element-hq/element-web#19629

[toger5]

Oh Fantastic!
Than I will close. Looking and learning from the threads code to get the timeline right panel card done.