Improve round corners in editor StyleBox

[hayahane]

Describe the project you are working on

Godot Editor

Describe the problem or limitation you are having in your project

Radius of round corners of some UI elements aren't unified.

• https://github.com/godotengine/godot/issues/95410 top round corners of audio bus ui
• left-top round corner of editor tabs
• SubViewportContainer:

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Apply editor round corner radius to these UI elements to get unified look.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Change the styleboxes generated for the ui elemens in editor code.

If this enhancement will not be used often, can it be worked around with a few lines of script?

No, this is written in editor theme in Godot source code.

Is there a reason why this should be core and not an add-on in the asset library?

It can be done in a hacky way, but it's complicated. This should be in editor theme code.

[Calinou]

The top corners of selected tabs are intentionally not rounded, as they look strange otherwise. The top border basically becomes tapered on its ends, which makes it look quite lopsided.

We could modify how borders are rendered on StyleBoxFlats with different border widths on each side, but there is no universal behavior that makes sense in all situations. This may mean we'd have to add yet another property in a StyleBoxFlat inspector that is already pretty complex.

[hayahane]

This is how unity deals with tabs with accent color. However this may need additional GUI styles beyond the capability of current Stylebox. Does this look like inner border?