patches/qtbase_5_12_8/0021-fallback-to-qgtk3-if-there-are-no-base-theme.patch

diff --git a/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.cpp b/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.cpp
index 30c43b67dc..08119609be 100644
--- a/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.cpp
+++ b/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.cpp
@@ -63,9 +63,11 @@ public:
     ~QXdgDesktopPortalThemePrivate()
     {
         delete baseTheme;
+        delete gtkTheme;
     }
 
     QPlatformTheme *baseTheme;
+    QPlatformTheme *gtkTheme;
     uint fileChooserPortalVersion = 0;
 };
 
@@ -92,12 +94,18 @@ QXdgDesktopPortalTheme::QXdgDesktopPortalTheme()
                 break;
         }
         // No error message; not having a theme plugin is allowed.
+
+        // Try to use gtk's open directory dialog if there are no fallback theme
+        d->gtkTheme = QPlatformThemeFactory::create(QLatin1String("gtk3"), nullptr);
     }
 
     // 3) Fall back on the built-in "null" platform theme.
     if (!d->baseTheme)
         d->baseTheme = new QPlatformTheme;
 
+    if (!d->gtkTheme)
+        d->gtkTheme = new QPlatformTheme;
+
     // Get information about portal version
     QDBusMessage message = QDBusMessage::createMethodCall(QLatin1String("org.freedesktop.portal.Desktop"),
                                                           QLatin1String("/org/freedesktop/portal/desktop"),
@@ -155,8 +163,12 @@ QPlatformDialogHelper* QXdgDesktopPortalTheme::createPlatformDialogHelper(Dialog
     if (type == FileDialog) {
         // Older versions of FileChooser portal don't support opening directories, therefore we fallback
         // to native file dialog opened inside the sandbox to open a directory.
-        if (d->fileChooserPortalVersion < 3 && d->baseTheme->usePlatformNativeDialog(type))
-            return new QXdgDesktopPortalFileDialog(static_cast<QPlatformFileDialogHelper*>(d->baseTheme->createPlatformDialogHelper(type)));
+        if (d->fileChooserPortalVersion < 3) {
+            if (d->baseTheme->usePlatformNativeDialog(type))
+                return new QXdgDesktopPortalFileDialog(static_cast<QPlatformFileDialogHelper*>(d->baseTheme->createPlatformDialogHelper(type)));
+            else if (d->gtkTheme->usePlatformNativeDialog(type))
+                return new QXdgDesktopPortalFileDialog(static_cast<QPlatformFileDialogHelper*>(d->gtkTheme->createPlatformDialogHelper(type)));
+        }
 
         return new QXdgDesktopPortalFileDialog;
     }