Non-root scrolling

[cfraz89]

Implements scrolling on mouse-wheel events, of any node which is hovered. When a node has hit its scroll boundary, scrolling bubbles up to the node's parent, all the way up to the root element. The root (html) element is sized to the viewport (is this technically the right thing to do?) and therefore will automatically handle root-level scrolling.

Adds an example scrolling which demos scrolling in the middle element. Scrolling the root element can be tested by resizing the window smaller than the content and scrolling, otherwise the tailwind example has a large content area and should be scrollable too.

Todo: Touch scrolling, flinging with momentum (even with mouse wheel? chromium does this).

[jkelleyrtp]

Thanks for the PR! Nico's currently on break so I'll take a look at it, but it'll take a moment :)

[cfraz89]

No problems

[cfraz89]

Thanks for the feedback, all makes sense. I revamped the PR:

• Scrolling behaviour is all implemented in document now
• Implemented the partial bubbling of scroll up to parent node, then to document scroll
• Removed locking root element size to viewport size, and instead track scroll on document again
• Flipped scroll direction
• use kurbo::Point instead of taffy::Point to store scroll coords, realised that taffy::Point has a bit of a css slant
• Fixed comment formatting