sway: fallback to legacy DRM interface on virtio

Some time ago, 'WLR_RENDERER=pixman' stopped working properly on virtio.
Appears that it only affects the atomic KMS and falling back to the
legacy DRM interface was enough to work around.

While legacy is a less tested codepath, I don't see any other ways to
address that :'(

initial-setup/run-gui-backend

@@ -41,10 +41,19 @@ chmod +x ${RUN_SCRIPT}
 
 
 # Set some compatibility variables if we're in a VM
-if systemd-detect-virt --quiet --vm; then
-    export WLR_NO_HARDWARE_CURSORS=1
-    export WLR_RENDERER=pixman
-fi
+case $(systemd-detect-virt --vm) in
+    "none"|"")
+        ;;
+    "kvm")
+        # WLR_NO_HARDWARE_CURSORS=1 is not needed with legacy DRM interface
+        export WLR_RENDERER=pixman
+        export WLR_DRM_NO_ATOMIC=1
+        ;;
+    *)
+        # https://github.com/swaywm/sway/issues/6581
+        export WLR_NO_HARDWARE_CURSORS=1
+        ;;
+esac
 
 sway --config=${CONFIG_FILE}
 exit_code=$(< ${EXIT_CODE_SAVE})

sddm/sddm-compositor-sway

@@ -12,9 +12,18 @@ if [ -x /usr/bin/start-sway ]; then
 fi
 
 # Set some compatibility variables in case if sway-config-fedora is not present
-if systemd-detect-virt --quiet --vm; then
-    export WLR_NO_HARDWARE_CURSORS=1
-    export WLR_RENDERER=pixman
-fi
+case $(systemd-detect-virt --vm) in
+    "none"|"")
+        ;;
+    "kvm")
+        # WLR_NO_HARDWARE_CURSORS=1 is not needed with legacy DRM interface
+        export WLR_RENDERER=pixman
+        export WLR_DRM_NO_ATOMIC=1
+        ;;
+    *)
+        # https://github.com/swaywm/sway/issues/6581
+        export WLR_NO_HARDWARE_CURSORS=1
+        ;;
+esac
 
 exec /usr/bin/sway "$@"

sway/start-sway

@@ -16,8 +16,6 @@ case $(systemd-detect-virt --vm) in
     "none"|"")
         ;;
     "kvm")
-        # https://github.com/swaywm/sway/issues/6581
-        export WLR_NO_HARDWARE_CURSORS=1
         # There's two drivers we can get here, depending on the 3D acceleration
         # flag state: either virtio_gpu/virgl or kms_swrast/llvmpipe.
         #
@@ -30,6 +28,10 @@ case $(systemd-detect-virt --vm) in
         #
         # See also: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/2871
         export WLR_RENDERER=pixman
+        # 'pixman' on virtio_gpu with recent kernels is glitchy. Appears that
+        # it only affects atomic KMS, and legacy interface works.
+        export WLR_DRM_NO_ATOMIC=1
+        # WLR_NO_HARDWARE_CURSORS=1 is not needed with legacy DRM interface
         ;;
     *)
         # https://github.com/swaywm/sway/issues/6581