Simple way to detect if pointer is over *any* egui painted shape

[Aceeri]

Describe the bug
There isn't a clear way to tell if the pointer is currently over an egui painted region of a window when using non-Area/area derived containers. bevy_mod_picking tries to detect when it is over an egui piece using the Context::is_pointer_over_area

To Reproduce
Create a CentralPanel, pointer only detects background.

Expected behavior

Screenshots

Desktop (please complete the following information):

• OS: Windows
• Version 0.20

Additional context

[lvaroqui]

See related comment here: Adanos020/egui_dock#95 (comment), the issue is same for egui_dock::DockArea and egui Panels.

[dmlary]

The issue is that a handful of bevy crates are using Context::is_pointer_over_area() early in the frame cycle, after input has been processed, but before any egui panels have been drawn. If the check is performed after any panels have been drawn, is_pointer_over_area() returns the correct results. But if you call it before drawing anything, it will always return false for Background layers.

is_pointer_over_area() is using FrameState.unused_rect.contains(...) to determine if the cursor position for Background layers falls within the layer:

egui/crates/egui/src/context.rs

Lines 1347 to 1349 in 9478e50

	if let Some(layer) = self.layer_id_at(pointer_pos) {
	if layer.order == Order::Background {
	!self.frame_state(|state| state.unused_rect.contains(pointer_pos))

The problem is that FrameState::begin_frame() resets FrameState.unused_rect to the full input.screen_rect() at the start of every frame.

egui/crates/egui/src/frame_state.rs

Line 97 in 9478e50

*unused_rect = input.screen_rect();

I can see why egui isn't just using the result from Context::layer_id_at(); the panels are all the same Layer as areas that don't contain egui windows.

Is the solution here to make Panels persistent layers similar to Windows?