Adding PositionType::Fixed

[Exanc]

Objective

Fixes #9564

Add PositionType::Fixed to help with the creation of popups.
"Fixed" nodes will be positioned relative to the windows and ignore every nodes in the graph.

Solution

When converting from bevy_ui::ui_node::PositionType::Fixed to taffy::style::Position the Absolute variant is used as their behavior is very similar (c.f.: MDN).

When the bevy_ui::layout::ui_layout_system system is ran, it keeps a set off all fixed_nodes to be used by update_uinode_geometry_recursive, subtracting a node's parent position from it's absolute position.

Changelog

• Added bevy_ui::ui_node::PositionType::Fixed

Notes

For now, PositionType::Fixed is relative to the windows but calculation of properties such as size or width are still done relative to the parent. Thus, not mimicking CSS's way off handling fixed objects, this can be counteracted by using Val::vh(...).

Some tests may be necessary, but I would like some feedback first on what exact effects of position: fixed we want to mimic.

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[Exanc]

After some thinking, mimicking more effects of position: fixed seems to be adding more unnecessary complexity, there are always ways around limitations for users and as said in the discord, were not trying to perfectly mimic CSS.