Add hints / guards against silent or unclear failures

Some things fail (or crash even) silently—we should add measures to inform users and possibly prevent the rendering.

https://floss.social/@sonny/109975391706946091

I discussed with James if this belongs in Blueprint or Workbench, no clear winner. Let's start/iterate in Workbench and possibly transfer to Blueprint later.

No child allowed

crash: no
priority: high

Passing children to objects that don't accept any such as Label will not provide any feedback. It is confusing to beginners as the children don't get rendered but no explanation is given.

We should emit an error.

Label {
  label: "Hello";
  Button {
    label: "World";
  }
}

Single child allowed

crash: no
priority: high

Some widgets like AdwStatusPage, Adw.Clamp, ScrolledWindow, ... allow only one child. It doesn't error but only the last child will be rendered. It's confusing and makes the user think they did something wrong.

OK

Adw.StatusPage {
  Button {
    label: "a";
  }
}

KO

Adw.StatusPage {
  Button {
    label: "a";
  }
  
  Button {
    label: "b";
  }
}

Order of properties on Gtk.Adjustment

crash: no
priority: medium

OK:

      Scale one {
        width-request: 130;
        adjustment: Gtk.Adjustment {
          lower: 0;
          upper: 100;
          value: 50;
        };
      }

KO:

      Scale one {
        width-request: 130;
        adjustment: Gtk.Adjustment {
          lower: 0;
          value: 50;
          upper: 100;
        };
      }

https://matrix.to/#/!aUhETchlgthwWVQzhi:matrix.org/$16780187509362SfqjZ:gnome.org?via=gnome.org&via=matrix.org&via=libera.chat

ToggleButton grouped to each others

Grouping ToggleButtons to each others will render fine but will cause Workbench to freeze when clicking on one of them.

crash: yes
priority: low

KO

using Gtk 4.0;
using Adw 1;

Box {
  halign: center;
  valign: center;
  orientation: horizontal;
  
  ToggleButton button_first {
    label: "Toggle On";
    group: button_second;
  }

  ToggleButton button_second {
    label: "Toggle Off";
    group: button_first;
  }
}

OK:

using Gtk 4.0;
using Adw 1;

Box {
  halign: center;
  valign: center;
  orientation: horizontal;
  
  ToggleButton button_first {
    label: "Toggle On";
  }

  ToggleButton button_second {
    label: "Toggle Off";
    group: button_first;
  }
}

icon not found

crash: no
priority: low

https://matrix.to/#/!kDBZrVKCdhrVuWxbGe:matrix.org/$GI0tPROepvgJBYCqjapML5GqxIXxS6bHukuFrj9RttQ?via=gnome.org&via=matrix.org&via=catgirl.cloud

some references does'nt show up and are replaced by a corrupted icon which looks odd

If we detect an icon as not fund - we could show a hint to lookup the icon library in the menu

Start by reproducing the listed Workbench cases: unsupported children, multiple children, Gtk.Adjustment property order, grouped ToggleButtons, and missing icons. Review how Workbench currently reports or handles these examples, then define focused behavior for each case; done should mean users receive clear feedback or the problematic rendering is prevented without freezes.