Add StickyContainer

[dugramen]

closes godotengine/godot-proposals#5599

2026-01-11.14-01-58.mp4

Adds StickyContainer. It allows sticking any children of StickyContainer to the bounds of the nearest ScrollContainer.

At runtime (not in the edited scene), when a child is added to StickyContainer, it gets reparented to the ScrollContainer, so that the controls appear above the normal scroll content and receive input first. The StickyContainer's rect is used to track where the sticky nodes would normally be, and then ScrollContainer handles actually positioning them, keeping it within its global rect

There's an optional bounding_container, which is a NodePath. If its set, the sticky control will also be limited to the bounds of that control, enabling those section headers.

Finally there's an optional stacked property, enabling the last example in the video.

There's a signal for "status_changed", telling when a sticky node is stuck, unstuck but out of view, or unstuck but in view. I'm not sure about these states, but it was what the linked proposal was mainly requesting