Cancel list scroll reset after mouse move on wheel scroll

[lmorchard]

Also fixes an error introduced in #9200 caused by attempting to use an
undefined prop.

Fixes #9217

[lmorchard]

I think this should fix #9217 - looked like the main issue was that I missed wheel scrolling as a source of intentional scrolling.

[mayaeh]

In my environment, I seems that scrolling the timeline with the Firefox for mac (63.0.1) is heavier than before merging this PR's code.

Edit: I felt it improved as I restarted the browser. I'm sorry.

I seems that there is no problem with the Safari for mac (12.0.1) and the Chrome for mac (70.0.3538.77).

As for Windows, I can not test it until I return from work... sorry.

I will try it in another environment after I return from work.

[Gargron]

I wonder if these things need to be in state if they are never rendered. Using simple variables might work better and improve performance.

[mayaeh]

I tried this on the Chrome for Windows (70.0.3538.77) and the Firefox for Windows (63.0.1).
I think that it was fixed within the range that I care about an issue.

[Gargron]

What's the extent of this issue if you are manually scrolling with page up/down, the scrollbar, or arrow keys? (Because the fix seems to depend on the wheel event specifically)

[lmorchard]

I wonder if these things need to be in state if they are never rendered

I'd put them in state because I figured I'd add some visual indication of the pause, but then figured it wasn't particularly necessary.

That can always be changed later, though, so I just amended the commit to move these into plain object properties. That simplifies the logic as a bonus.

[lmorchard]

And one more tweak, as I just realized my call to this.props.scrollToTop was redundant because the scroll event gets fired by setting node.scrollTop and handleScroll takes care of it anyway.