transparent StackView layers

[dermetfan]

In response to #117 (comment) I removed the Layer from StackView, added a background to Dialog and Panel, and adapted usages. On the caller's side that leads to the following situation:

add_layer(Layer::new(…))

By default there will be no background so in its current state this silently breaks many applications and is not very intuitive / easy to do wrong. You can see how many occurrences I had to update in the examples. Because the background is not cleared this also causes artifacts when scrolling text.

Maybe we should keep the default behavior and just add something like add_transparent_layer()? I'd like to agree on the API changes before I go ahead.

[gyscos]

Thank you very much for the work!

You're right, silently changing add_layer to be transparent might be surprising; a add_transparent_layer sounds like a better solution.

A simple solution would be, from your PR, to simply rename add_layer to add_layer_transparent, and add something like

fn add_layer<T>(&mut self, view: T)
    where T: IntoBoxedView,
{
    self.add_transparent_layer(Layer::new(view));
}

Unfortunately, thinking about it more, we have the same problem we had with the optional ShadowView wrapper: we want users who pop layers to get the View they inserted. So if I do add_layer(MyView::new()), I want pop_layer to return this MyView, without being wrapped by a Layer or ShadowView.

The solution is the same we had back then: add a variant to the ChildWrapper enum, so that it could be either Shadow(ShadowView<Layer<T>>), Backfilled(Layer<T>), or Transparent(T).

This means adding add_transparent_layer and add_transparent_layer_at.

This may sound like an uncontrolled explosion in methods or variants, but:

• The variants for ChildWrapper are bounded: we did not add a new wrapper, we removed the last mandatory wrapper.
• We could remove add_layer_at and only keep add_transparent_layer_at: it's an advanced API to give more control; we can expect users in this case to be fine with manually wrapping a view in a Layer.

Going this road, I don't think it makes sense to backfill in Dialogs or Panels anymore.

[dermetfan]

That turned out pretty well, only StackView is modified now. Thanks for the input!
I also added transparent_layer() for a chainable variant.

[dermetfan]

(Kept the Plain name for now, I think it's still a good fit)

[gyscos]

I think we still want the absolute inner view here, so it would be get_inner().get_inner() (just like before this PR)?

[gyscos]

Same thing here, any reason you removed a round of get_inner_mut()?

[dermetfan]

No, those are my fault, it seems I haven't properly restored everything after the previous attempt.

[dermetfan]

Good catch! I added a test that would have caught my mistake.

[gyscos]

Awesome, thanks!