Set default scrollbar-width to scroll bar

[luixxiul]

Description:

Set default scrollbar-width to scroll bar. It is reported on the upstream that grabbing resizer on the main timeline next to the thread panel is very hard. MDN also says that you would have to be careful to set thin value from the perspective of accessibility (https://developer.mozilla.org/en-US/docs/Web/CSS/scrollbar-width#accessibility_concerns). Basically, there is no necessity to set thin value to the scroll bar here at all.

Now that the issue was originally reported in 2022 and is still open, it seems that the upstream project is not really interested in improving it (also, the way how Firefox renders thin scroll bar itself seems to have been improved since then), and I think we can just go ahead.

The scrollbar is already so thin (element-hq/element-web#22084) on Firefox and already difficult to grab, and it gets even more difficult due to a conflict with the panel width change function. Maybe only 1~2 pixel is available for grabbing.

scrollbar.mp4

Describe alternatives you've considered:

Additional context:

[github-actions]

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