Collapsing header improvements

[DrOptix]

This is a PR without a backing issue.

This PR changes CollapsingHeader container like this:

• CollapsingHeader is now opening or closing only when clicking the arrow icon
• CollapsingHeader is now filling all the available horizontal space
• Emit clicks on CollapsingHeader's header only when NOT clicking the icon

Visual display:

[emilk]

I think being able to click the entire header should be the default behavior, just as it is the default behavior to be able to click the label of a checkbox or a radiobutton to toggle it. This new behavior is only useful for selectable collapsing headers, so I suggest we restrict it to those.

[coderedart]

just a random suggestion: single click could select and double click (or long press/click) could be expand. kinda like reddit comment threads or some tree ui in android apps.

[DrOptix]

I think being able to click the entire header should be the default behavior, just as it is the default behavior to be able to click the label of a checkbox or a radiobutton to toggle it. This new behavior is only useful for selectable collapsing headers, so I suggest we restrict it to those.

Now that you mention it my target is to be able to create something like this for my project.

Now I'm not quite sure how to make it API wise so we can add custom controls to the header. In my case I would like to add a checkbox in front of the title and a button on the right side of the label as it can be seen in the screenshot above.

I'm thinking of adding something like this, but the drawback is that you need to paint the title yourself:

CollapsingHeader::new("title").show_custom_header(|title, ui| {
    // Show checkbox

    // Show title

    // On the right side of the header, show button for actions popup menu
});

[emilk]

@DrOptix I think the best solution for that screenshot is to refactor CollapsingHeader so one can have a CollapsingSection without a header, controlled by a button. This is sort of how Window implements collapsing.

[emilk]

This change produces some weirdness:

I wonder if it is is better to try to make the CollapsingHeader more composable instead, so that it is easier to create separate functions for the old-style collapsing header, and your "separate arrow and header". As I mentioned before, Window already has a collapsing arrow as a separate widget, and I suspect we could make that even more reusable and composable.

But then again, composability is difficult in immediate mode.

[DrOptix]

I need to get back on this as I put it on the backlog for quite some time.

[emilk]

I found a nicer approach in #1538