Allow ScrollContainer to be clicked for focus and border

[ator-dev]

The SceneTree and FileSystem docks have focus borders automatically since TreeDialog is one of the few classes which provides this, and also offer focus-on-click. In my opinion the style of this works well with Godot and is potentially useful for checking what the user is currently doing - for example, if they are editing the SceneTree then that dock has focus.

In order to make the Inspector dock more consistent with this style, since it has a similar feel and context anyway, I have made it (a) clickable and (b) draw the same border when focused. This is a really useful feature for another PR I'm making soon which needs to recognise what dock the user is working in, but it certainly has other use cases (e.g. for plugins) as well and just looks nice.

Problems:

1. The border-focus stylebox is drawn underneath elements such as text boxes. I don't know if this matters, but it's something that could be fixed if necessary. This has now been fixed by using a more general and standard method for drawing the focus stylebox.
   
2. When doing something like dragging a slider immediately after something else was focused, focus returns to that area and not the Inspector. This probably doesn't matter though and, again, could be fixed. This still occurs but is no longer a problem, since recent updates have made focus changes more stable.

Update 2022-09-01: additionally, the focus border has been made available in ScrollContainer as a whole.

[ator-dev]

Fullscreen image for reference:

[YuriSizov]

We discussed this in a PR meeting. This is probably good, needs an approving review.

[ator-dev]

This has now been generalised in order to make the focus border available in any instance of ScrollContainer. I hope this is OK as I didn't make a proposal about it, but since this behaviour is consistent with Tree and ItemList (using the focus_bg theme property) I think it's a reasonable expansion that should be useful to Godot users and developers.

As a consequence, many more editor docks/panels now have focus borders which makes the editor more consistent and accessible.

[KoBeWi]

Found a bug in TileSet editor:

Other than that it looks good.

Here's a rebased patch:
0001-scroll.patch.txt

[KoBeWi]

Something went wrong and I became author of the commit.

[ator-dev]

That's weird, I suppose it was the way I applied the patch. Does it matter?

[ator-dev]

By the way, I have addressed your issue with the latest push. I thought that part of the TileSet editor was inconsistent with the rest of Godot anyway, and it could be unclear to users what the panel is if they scroll the header out of view. As you can see I have turned it into a proper scrollable sub-panel (separate case for each TileSet tool):

[KoBeWi]

I think the problem is that you are ignoring clipping for the focus style. Even if you fix tile editor, this makes a generic problem with nested scroll containers and there might be other broken places.

I suppose it was the way I applied the patch. Does it matter?

Probably. There is a way to change the author though. In worst case you can manually modify the patch.

[ator-dev]

this makes a generic problem with nested scroll containers

That's true - as far as I can tell it happens with Tree too, the focus behaviour of which I was replicating, but I haven't tested fully yet because I find Godot's UI nodes hard to use. I'm not sure yet if it can be easily fixed by using a more appropriate z-index.

[ator-dev]

I just realised what you meant about ignoring clipping - yes, this is a problem and this solution is more of a hack than a proper fix. I'm not sure how else to get the focus border to show up.

[ator-dev]

New state of the TileSet editor, with changes to improve the scrolling behaviour:

Please let me know if this is big enough to be separated into its own PR.

[ator-dev]

The default theme no longer includes borders for ScrollContainers so there should not be a general problem with the slightly hacky method I used.

[pafuent]

@ator-dev If it's fine with you, I can try to implement this on master
Of course I'll add you as Co-Author, because most of the code will be yours

[ator-dev]

Of course, I'd be delighted to see this merged! Go ahead @pafuent :)

[akien-mga]

Superseded by #97521.