My app, when running inside Flatpak with org.gnome.Platform v43 runtime, its header bar (as HdyHeaderBar widget) becomes too big. I don’t know what causes it. Seem not to be libhandy version, because when running outside Flatpak, in Ubuntu 22.10 (which comes with libhandy 1.8), the header bar size is still normal.
Here is what it looks when running inside Flatpak:
Check flatpak for updates (for org.gtk3theme.adw-gtk3 specifically). If there isn’t any update available, you will have to change your GTK3 (Legacy Applications) theme to something else, preferebly the default one.
It looks like the problem is related to the icon theme, in particular the webcam icon. For some reason it’s bigger than the other icons in flatpak, and bumps up the (minimum) size of the whole headerbar.