# Objective
Allow UI elements to be positioned relative to the viewport rather than
their parent element.
Fixes#9564
## Solution
- New UI marker component `FixedNode`. Requires `Node` and
`OverrideClip`.
- `FixedNode` entities are treated as UI roots in layout, even if they
have a parent.
- `FixedNode`s don't inherit their parent's layout, clipping or
transform context.
- During the taffy layout updates children with `FixedNode` are skipped.
- Added a couple of basic helper functions to `UiSurface`, mainly there
to make the tests a little less painful.
- Added a fairly comprehensive range of new tests, including tests with
`GhostNode`s.
- In the Taffy layout (stored in `UiSurface`) there is nothing to
distinguish `FixedNode`s and root nodes, so they are treated identically
during updates.
--
The original suggestion was to implement it as a `PositionType::Fixed`
variant that could used with `Node`, but I think that would be much more
complicated without support from Taffy. Being able to just directly
query for and filter out `FixedNode` entities directly makes the
implementation much simpler and more efficient.
## Testing
Basic example which shows events bubbling up to the parent from the
fixed node:
```
cargo run --example fixed_node
```
There are also a number of new tests in the `layout` module.
```
cargo test -p bevy_ui --lib --features ghost_nodes
```
---------
Co-authored-by: François Mockers <francois.mockers@vleue.com>
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