Xdg-desktop-portal upgrade breaks GTK apps

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To summarize the issue, this problem was effectively introduced by xdg-desktop-portal 1.18.0, as it was mentioned in the release note

Changes that might be relevant for distributors:

  • Rework how portal implementations are loaded. This new, more robust system allows selecting specific backends for specific portals, and layering them when necessary. Platforms that provide portals implementation are encouraged to provide a suitable configuration file.

See [Bug]: Flatpak doesn't use Breeze theme, uses Adwaita in KDE Plasma · Issue #1112 · flatpak/xdg-desktop-portal · GitHub

So on KDE, you must install xdg-desktop-portal, xdg-desktop-portal-kde and xdg-desktop-portal-gtk, the latest package is used as fallback for non-native GTK apps (Flatpaks, Appimages…) to fix the problem related to antialiasing font and themes on KDE Plasma.

The fix of fallback order was already made by KDE and was backported to 5.27, if you need quick fix without waiting for the fix you simply edit /usr/share/xdg-desktop-portal/kde-portals.conf like the following:

[preferred]
default=kde
org.freedesktop.impl.portal.Settings=kde;gtk;

After rebooting, GTK_USE_PORTAL=1 will work again like it did before.

Edit: upgrading to xdg-desktop-portal-kde 5.27.8-3 fixes the issue.

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