Add manual scrolling

[lucaspoffo]

Fixes #74

[lucaspoffo]

Added the scroll_to_me inside Response,
Needed to pass the center_factor to the Context, so we can correctly centralize inside the ScrollArea.

[lucaspoffo]

Update the scroll demo (made it similar to imgui) and moved it to its own module.
Added documentation to the methods.
Added ScrollArea::scroll_offset().

[emilk]

Very nice! Thanks for taking your time to work on this.

I left a lot of comments. Most contentious will probably be center_factor vs Align. This is basically a tradeoff between flexibility and simplicity, and I'm keen to hear your thoughts on it!

[emilk]

Suggested change

	/// * 1.0 - bottom
	/// * 1.0 - bottom
	///
	/// ```
	/// # let mut ui = egui::Ui::__test();
	/// egui::ScrollArea::auto_sized().show(ui, |ui| {
	/// for i in 0..1000 {
	/// if ui.button(format!("Button {}", i)).clicked {
	/// ui.scroll_to_here(0.5);
	/// }
	/// }
	/// });
	/// ```

[emilk]

...though in this case, this would make more sense:

let response = ui.button(format!("Button {}", i));
if response.clicked {
    response.scroll_to_me(0.5);
}

Is there any other more plausible use case of this function?

[lucaspoffo]

I changed the code example from the scroll_to_cursor to use the scroll_bottom example similar from the demo. Makes more sense for this case. Added the button example for the scroll_to_me.

[emilk]

This will scroll to the bottom of the current size of the Ui, not to the current cursor position (a Ui can be resized to something large before adding elements to it).

I think scroll_to_cursor and let scroll_target = self.region.cursor.y; is better

[lucaspoffo]

Good point 👍 Didn't knew about that.

[emilk]

Another minor addition: when clicking a button to scroll to the top of that button, I think it would make sense to add a small offset (e.g. item_spacing.y) to the scroll. Otherwise the top of the button frame will be clipped by the scroll areas clip rect. In other words, it will look better if the top and bottom scrolls always came with an offset of item_spacing.y.

[emilk]

Suggested change

	let height_offset = content_ui.clip_rect().height() * center_ratio;
	let top = content_ui.min_rect().top();
	let offset_y = scroll_target - top - height_offset;
	let offset_y = scroll_target - lerp(content_ui.clip_rect().y_range(), center_ratio);

[lucaspoffo]

This does not work, as the clip_rect is always the same, but I updated the code to be more easy to understand. I hope.

[lucaspoffo]

I don't know if this is the best place to add this.

[emilk]

This is the right place! Maybe could be called simply to_factor

[emilk]

Good example 👍

[emilk]

Awesome - this works great!

Thank you!