Missing toolbar/menu icon in Flatpak build under XFCE/Cinnamon (GTK4/libadwaita rendering issue?)

[CipherCloak]

Summary

One toolbar/menu icon is rendered as a blank gray placeholder instead of the expected symbolic icon.

The issue happens only with Foliate (Flatpak version) and appears consistently across multiple desktop environments.

This does not seem to be caused by missing icon themes or Flatpak icon access permissions.

Screenshot

The missing icon is the one highlighted in red:

Environment

• Foliate installed from Flatpak

• Desktop environments tested:

  • XFCE 4.18
  • XFCE 4.20
  • Cinnamon

• GTK4/libadwaita application

• Problem reproducible across different icon themes

What I already tested

Icon theme access for Flatpak

I exposed system icons to Flatpak:

flatpak override --user --filesystem=/usr/share/icons:ro
flatpak override --user --filesystem=xdg-data/icons:ro

I also tested:

ln -s /usr/share/icons/* ~/.icons/
flatpak override --user --filesystem=~/.icons:ro

No change.

---

Installed Adwaita icon themes

Installed and tested with:

• adwaita-icon-theme
• org.gtk.Gtk3theme.Adwaita

No change.

---

Forced Adwaita theme

Tested with:

GTK_THEME=Adwaita flatpak run com.github.johnfactotum.Foliate

No change.

Also tested:

flatpak run --env=ICON_THEME=Adwaita com.github.johnfactotum.Foliate

No change.

---

Removed GTK4 overrides

Removed/renamed:

~/.config/gtk-4.0

No change.

---

Backend tests

Tested both:

GDK_BACKEND=x11

and

GDK_BACKEND=wayland

No difference.

Debug output

Running:

flatpak run --command=sh com.github.johnfactotum.Foliate
GTK_DEBUG=interactive foliate

does NOT show missing icon warnings such as:

Failed to load icon

Instead, GTK reports layout/allocation warnings:

Gtk-CRITICAL **: gtk_widget_measure: assertion 'for_size >= -1' failed

Gtk-WARNING **: gtk_widget_size_allocate(): attempt to allocate FoliateBookItem ... with width -30 and height -18

This makes me suspect the icon/widget exists but is being allocated an invalid size, resulting in the gray placeholder rendering.

Additional notes

The issue looks more like a GTK4/libadwaita rendering/layout problem than a missing icon asset problem.

Could this be related to symbolic icon sizing/allocation under non-GNOME desktop environments?