# Objective
#### Goals
* Stop layout updates from overwriting `ScrollPosition`.
* Make `ScrollPosition` respect scale factor.
* Automatically allocate space for a scrollbar on an axis when
`OverflowAxis::Scroll` is set.
#### Non-Goals
* Overflow-auto support (I was certain Taffy had this already, but
apparently I was hallucinating).
* Implement any sort of scrollbar widgets.
* Stability (not needed because no overflow-auto support).
* Maybe in the future we could make a `ScrollbarWidth` enum to more
closely match the CSS API with its auto/narrow/none options. For now
`scrollbar_width` is just an `f32` which matches Taffy's API.
## Solution
* Layout updates no longer overwrite `ScrollPosition`'s value.
* Added the field `scrollbar_width: f32` to `Node`. This is sent to
`Taffy` which will automatically allocate space for scrollbars with this
width in the layout as needed.
* Added the fields `scrollbar_width: f32` and `scroll_position: Vec2` to
`ComputedNode`. These are updated automatically during layout.
* `ScrollPosition` now respects scale factor.
* `ScrollPosition` is no longer automatically added to every UI node
entity by `ui_layout_system`. If every node needs it, it should just be
required by (or be a field on) `Node`. Not sure if that's necessary or
not.
## Testing
For testing you can look at:
* The `scrollbars` example, which should work as before.
* The new example `drag_to_scroll`.
* The `scroll` example which automatically allocates space for
scrollbars on the left hand scrolling list. Did not implement actual
scrollbars so you'll just see a gap atm.
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Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
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