FixedNode

[ickshonpe]

Objective

Allow UI elements to be positioned relative to the window rather than its parent element.

Fixes #9564

Solution

New UI marker component FixedNode.
FixedNode entities are treated as UI roots in layout, even if they have a parent.
During the taffy layout updates children with FixedNode are skipped.

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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 everything much simpler.

Testing

Basic example which shows events bubbling up to the parent from the fixed node:

cargo run --example fixed_node

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I need to write some tests to make sure the layout isn't broken when FixedNodes are removed or reparented etc, but otherwise it seems like a reasonable approach.

[viridia]

Release note please :)

[amtep]

What should happen if a root node has FixedNode? I think right now the code makes the assumption that the sets are disjoint.

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[ickshonpe]

What should happen if a root node has FixedNode? I think right now the code makes the assumption that the sets are disjoint.

Hopefully just adding a Without<Children> filter to the FixedNode queries will be sufficient. GhostNodes might cause some problems though. I'll add some comprehensive tests tomorrow.