[Flatpak] White border on GTK apps installed from flathub repository

[tur1ngb0x]

Describe the problem: White border on the top of window decorations for all GTK apps installed from flathub repository.

Screenshots: Nautilus, Gnome Boxes, Okular Nautilus, Flatseal, VLC

Steps to reproduce:

1. Launch Nautilus.
2. Launch Gnome Boxes flatpak. (or any GTK flatpak app from flathub)
3. Launch Okular flatpak. (or any QT flatpak app from flathub)
4. GTK flatpak apps have a one pixel white border on the top of window decoration.

Distribution:

$ cat /etc/os-release | grep -i pretty_name
PRETTY_NAME="Pop!_OS 20.04 LTS"

PopOS Version:

$ apt-cache policy pop-gtk-theme | grep -i installed
Installed: 5.2.0~1620141353~20.04~2f38cad

$ flatpak info org.gtk.Gtk3theme.Pop
Pop Gtk theme - Pop Gtk theme used by System76

          ID: org.gtk.Gtk3theme.Pop
         Ref: runtime/org.gtk.Gtk3theme.Pop/x86_64/3.22
        Arch: x86_64
      Branch: 3.22
      Origin: flathub
  Collection: org.flathub.Stable
Installation: user
   Installed: 1.1 MB

      Commit: 1eac06f6a58047fd6717a13f12b744ea59e6f9829fed02d856a5efb998b318a5
      Parent: bf0014cc176fe2a66eb69045bc3ed80663e09240dda1db97b5394ba3a501bea3
     Subject: add flathub.json (84e0033a)
        Date: 2020-11-17 21:00:45 +0000

[cassidyjames]

I'm also noticing this when testing AppCenter with the Pop! theme locally. It doesn't show up in Pop-dark, just Pop.

[isantop]

I believe this is related to the HdyWindow issues. These should be fixed once the new Impish version of the theme is finalized and released to Flathub.