Sometimes you want to answer the question "What is this Ui part of?". For instance: sometimes you want to paint the background of a widget to extend to the edges of the panel it is in, but that panel may be several Uis removed from the current widget.

What you want is access to a stack of parent Ui frames, with some semantics.

Each stack should contain:

• Id
• Frame (inner & outer margins, stroke, …)
• min_size and max_size (with/without margins)
• Type (Panel, Area, Group, …)

We would push onto this stack each time we create a new Ui.

This means the root of the stack is created by Ui::new (most often created by Area/Window and Panel).
Then things like ui.group, ui.horizontal, egui::Frame etc would push onto this stack before calling into its contents, and then pop afterwards.

Working name: StackFrame.

Then we add impl Ui { pub fn stack(&self) -> &Stack { … } } and then users can walk the stack to implement advanced styling.

Implementation

Ui::new creates the stack as a Rc<RefCell<Stack>> and hands it to its children.

Future work: columns

We can use this architecture to implement columnar layouts, allowing aligning widgets that share an ancestor.
Each StackFrame has an optional field for column widths.

An egui::Grid would create such a special stack frame.
When adding a widget, we would walk up the stack to find which column we are in, and align to that, then register if we overflowed that width.
ui.end_row() would reset a column=0.

Maybe this should be a separate stack? Probably.

Anyway, this requires more design, so let's not discuss this further here.