We now always have a RenderContext when rendering MouseEventHandlers or scrollable Flex columns/rows.
Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>
Previously, we were mutating the remaining space stored on the layout
state, which would cause re-paints to always have a `remaining_space`
of 0 and therefore not align `flex_float` elements to the right/bottom.
This is in contrast to not dispatching the event altogether in `Flex` when
the event is not contained in the flex element bounds. That approach was
problematic because it didn't give an opportunity to `MouseEventHandler`s
to handle mouse move events when they didn't intersect with the element bounds,
causing elements to never clear their hover state, cursor style, etc.
This adds the ability to make a Flex element scrollable by passing a type tag and instance id, which we use to store the scroll position in an ElementStateHandle.
Still need to allow the element to auto-scroll.
Keith Simmons
Nathan SoboandMax Brunsfeld
