Created attachment 169292 [details]
Simple UI file with 5 menu buttons

SUMMARY
GtkMenuButton's arrows are not shown at all.
From https://docs.gtk.org/gtk4/class.MenuButton.html#css-nodes

> Inside the toggle button content, there is an arrow node for the indicator, which will carry one of the .none, .up, .down,  > .left or .right style classes to indicate the direction that the menu will appear in. The CSS is expected to provide a 
> suitable image for each of these cases using the -gtk-icon-source property.

Something like the following should be added to gtk4's css file:
```
menubutton arrow.none { -gtk-icon-source: -gtk-icontheme("open-menu-symbolic"); }
menubutton arrow.down { -gtk-icon-source: -gtk-icontheme("pan-down-symbolic"); }
menubutton arrow.up { -gtk-icon-source: -gtk-icontheme("pan-up-symbolic"); }
menubutton arrow.left { -gtk-icon-source: -gtk-icontheme("pan-start-symbolic"); }
menubutton arrow.right { -gtk-icon-source: -gtk-icontheme("pan-end-symbolic"); }
```

STEPS TO REPRODUCE
1. Download the test.ui file attached. It contains a GtkWindow and 5 GtkMenuButtons, one in each direction.
2. Run `gtk4-builder-tool preview test.ui` with gtk theme set to Breeze in KCM.
3. See empty buttons

OBSERVED RESULT
Empty buttons

EXPECTED RESULT
Directional indicators in the buttons