fix(QScrollArea): prevent content re-rendering on scroll or mousemove (fix #16579)

[thexeos]

What kind of change does this PR introduce?

• Bugfix

Does this PR introduce a breaking change?

• No

The PR fulfills these requirements:

• It's submitted to the dev branch (or v[X] branch)
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on an Electron app

Other information:

After separating out the thumb rendering logic into a separate component we can isolate the rendering when scroll position is updated or thumb is shown/hidden.

The HTML and CSS output remains unchanged. The functionality remains unchanged. UI tests under ui/dev (ui/playground) all appear to be functioning as expected.

Before the change

After the change

This would fix #16579

[thexeos]

I found an edge case where it still triggers re-rendering. So this requires a little more work.

[github-actions]

UI Tests Results

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Results for commit d9e2a5f.

♻️ This comment has been updated with latest results.

[thexeos]

It turns out that Vue behaves slightly differently in dev and prod modes. It seems the effect isolation is more strict in dev and so it was not triggering the re-rendering of scrollable content when mainStyle was re-computed after hover ref was updated in @mouseenter and @mouseleave event handlers.

A complete solution is not possible without changing the code flow, as QScrollArea render function ultimately depends on scroll.vertical.thumbHidden and scroll.horizontal.thumbHidden values, which in turn change when showing/hiding the scrollbars in "default" visible mode.

Despite not being a complete solution, this change achieves:

• No more pointless re-rendering of content when scrolling [had the largest affect on performance]
• No more pointless re-rendering of content when showing/hiding the scrollbars, as long as contentStyle or contentActiveStyle props of QScrollArea were not set

This PR should be ready for merging.