wayland: Wire up wp_fractional_scale

src/wayland/handlers/fractional_scale.rs

@@ -0,0 +1,60 @@
+use crate::{state::State, utils::prelude::SeatExt};
+use smithay::{
+    delegate_fractional_scale,
+    desktop::utils::surface_primary_scanout_output,
+    reexports::wayland_server::protocol::wl_surface::WlSurface,
+    wayland::{
+        compositor::{get_parent, with_states},
+        fractional_scale::{with_fractional_scale, FractionalScaleHandler},
+    },
+};
+
+impl FractionalScaleHandler for State {
+    fn new_fractional_scale(&mut self, surface: WlSurface) {
+        // Here we can set the initial fractional scale
+        //
+        // First we look if the surface already has a primary scan-out output, if not
+        // we test if the surface is a subsurface and try to use the primary scan-out output
+        // of the root surface. If the root also has no primary scan-out output we just try
+        // to use the first output of the toplevel.
+        // If the surface is the root we also try to use the first output of the toplevel.
+        //
+        // If all the above tests do not lead to a output we just use the active output
+        // of the last active seat (which will also be the output a toplevel will
+        // initially be placed on)
+        let mut root = surface.clone();
+        while let Some(parent) = get_parent(&root) {
+            root = parent;
+        }
+
+        with_states(&surface, |states| {
+            let output = surface_primary_scanout_output(&surface, states)
+                .or_else(|| {
+                    if root != surface {
+                        with_states(&root, |states| {
+                            surface_primary_scanout_output(&root, states).or_else(|| {
+                                self.common
+                                    .shell
+                                    .visible_outputs_for_surface(&surface)
+                                    .next()
+                            })
+                        })
+                    } else {
+                        self.common
+                            .shell
+                            .visible_outputs_for_surface(&surface)
+                            .next()
+                    }
+                })
+                .unwrap_or_else(|| {
+                    let seat = self.common.last_active_seat();
+                    seat.active_output()
+                });
+            with_fractional_scale(states, |fractional_scale| {
+                fractional_scale.set_preferred_scale(output.current_scale().fractional_scale());
+            });
+        });
+    }
+}
+
+delegate_fractional_scale!(State);

src/wayland/handlers/mod.rs

@@ -5,6 +5,7 @@ pub mod compositor;
 pub mod data_device;
 pub mod decoration;
 pub mod dmabuf;
+pub mod fractional_scale;
 pub mod keyboard_shortcuts_inhibit;
 pub mod layer_shell;
 pub mod output;