ComboBox select dialog goes offscreen whenever it is opened too low

[TheSylex]

Whenever you position a combobox too low on the screen, its select window goes out of the window, like this:

Instead it should move upwards to fit.

[DrOptix]

For this one I think me need that layout size pre-process mentioned here in #606.

I'm facing the same issue while trying to use popup::popup_below_widget to show a popup when I click a button.
Seeing that ComboBox uses popup internally I tried to fix it.

The idea that I have is to compare ui.input().screen_rect() to the rect generated by the Area from here:

egui/egui/src/containers/popup.rs

Lines 275 to 310 in 5539dbe

	pub fn popup_below_widget<R>(
	ui: &Ui,
	popup_id: Id,
	widget_response: &Response,
	add_contents: impl FnOnce(&mut Ui) -> R,
	) -> Option<R> {
	if ui.memory().is_popup_open(popup_id) {
	let parent_clip_rect = ui.clip_rect();
	let inner = Area::new(popup_id)
	.order(Order::Foreground)
	.fixed_pos(widget_response.rect.left_bottom())
	.show(ui.ctx(), |ui| {
	ui.set_clip_rect(parent_clip_rect); // for when the combo-box is in a scroll area.
	let frame = Frame::popup(ui.style());
	let frame_margin = frame.margin;
	frame
	.show(ui, |ui| {
	ui.with_layout(Layout::top_down_justified(Align::LEFT), |ui| {
	ui.set_width(widget_response.rect.width() - 2.0 * frame_margin.x);
	add_contents(ui)
	})
	.inner
	})
	.inner
	})
	.inner;
	if ui.input().key_pressed(Key::Escape) || widget_response.clicked_elsewhere() {
	ui.memory().close_popup();
	}
	Some(inner)
	} else {
	None
	}
	}

[bonsairobo]

This seems possible without #606. One option is to choose the popup direction based on the y coordinates of box. Upper half of the window means open downwards, lower half means open upwards.