On Wayland, add support for fractional scaling

This adds support for the fractional scaling on Wayland via the
wp-fractional-scale protocol.

Co-authored-by: Julian Orth <ju.orth@gmail.com>

CHANGELOG.md

@@ -59,6 +59,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 - On X11, added `drag_resize_window` method.
 - Added `Window::set_transparent` to provide a hint about transparency of the window on Wayland and macOS.
 - On macOS, fix the mouse buttons other than left/right/middle being reported as middle.
+- On Wayland, support fractional scaling via the wp-fractional-scale protocol.
 
 # 0.27.5
 

Cargo.toml

@@ -37,7 +37,7 @@ rustdoc-args = ["--cfg", "docsrs"]
 [features]
 default = ["x11", "wayland", "wayland-dlopen", "wayland-csd-adwaita"]
 x11 = ["x11-dl", "mio", "percent-encoding"]
-wayland = ["wayland-client", "wayland-protocols", "sctk"]
+wayland = ["wayland-client", "wayland-protocols", "sctk", "wayland-commons"]
 wayland-dlopen = ["sctk/dlopen", "wayland-client/dlopen"]
 wayland-csd-adwaita = ["sctk-adwaita", "sctk-adwaita/ab_glyph"]
 wayland-csd-adwaita-crossfont = ["sctk-adwaita", "sctk-adwaita/crossfont"]
@@ -109,14 +109,18 @@ mio = { version = "0.8", features = ["os-ext"], optional = true }
 percent-encoding = { version = "2.0", optional = true }
 sctk = { package = "smithay-client-toolkit", version = "0.16.0", default_features = false, features = ["calloop"],  optional = true }
 sctk-adwaita = { version = "0.5.1", default_features = false, optional = true }
-wayland-client = { version = "0.29.4", default_features = false,  features = ["use_system_lib"], optional = true }
-wayland-protocols = { version = "0.29.4", features = [ "staging_protocols"], optional = true }
+wayland-client = { version = "0.29.5", default_features = false,  features = ["use_system_lib"], optional = true }
+wayland-protocols = { version = "0.29.5", features = [ "staging_protocols"], optional = true }
+wayland-commons = { version = "0.29.5", optional = true }
 x11-dl = { version = "2.18.5", optional = true }
 
 [target.'cfg(target_os = "redox")'.dependencies]
 orbclient = { version = "0.3.42", default-features = false }
 redox_syscall = "0.3"
 
+[target.'cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd", target_os = "openbsd", target_os = "netbsd"))'.build-dependencies]
+wayland-scanner = "0.29.5"
+
 [target.'cfg(target_arch = "wasm32")'.dependencies.web_sys]
 package = "web-sys"
 version = "0.3.22"

build.rs

@@ -1,8 +1,34 @@
 use cfg_aliases::cfg_aliases;
 
+#[cfg(all(
+    any(
+        target_os = "linux",
+        target_os = "dragonfly",
+        target_os = "freebsd",
+        target_os = "openbsd",
+        target_os = "netbsd",
+    ),
+    feature = "wayland",
+))]
+mod wayland {
+    use std::env;
+    use std::path::PathBuf;
+    use wayland_scanner::Side;
+
+    pub fn main() {
+        let mut path = PathBuf::from(env::var("OUT_DIR").unwrap());
+        path.push("fractional_scale_v1.rs");
+        wayland_scanner::generate_code(
+            "wayland_protocols/fractional-scale-v1.xml",
+            &path,
+            Side::Client,
+        );
+    }
+}
+
 fn main() {
     // The script doesn't depend on our code
-    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed=build.rs:wayland_protocols");
     // Setup cfg aliases
     cfg_aliases! {
         // Systems.
@@ -20,4 +46,17 @@ fn main() {
         wayland_platform: { all(feature = "wayland", free_unix, not(wasm), not(redox)) },
         orbital_platform: { redox },
     }
+
+    // XXX aliases are not available for the build script itself.
+    #[cfg(all(
+        any(
+            target_os = "linux",
+            target_os = "dragonfly",
+            target_os = "freebsd",
+            target_os = "openbsd",
+            target_os = "netbsd",
+        ),
+        feature = "wayland",
+    ))]
+    wayland::main();
 }

src/platform_impl/linux/mod.rs

@@ -441,7 +441,7 @@ impl Window {
 
     #[inline]
     pub fn scale_factor(&self) -> f64 {
-        x11_or_wayland!(match self; Window(w) => w.scale_factor() as _)
+        x11_or_wayland!(match self; Window(w) => w.scale_factor())
     }
 
     #[inline]

src/platform_impl/linux/wayland/env.rs

@@ -14,6 +14,7 @@ use sctk::reexports::protocols::unstable::relative_pointer::v1::client::zwp_rela
 use sctk::reexports::protocols::unstable::pointer_constraints::v1::client::zwp_pointer_constraints_v1::ZwpPointerConstraintsV1;
 use sctk::reexports::protocols::unstable::text_input::v3::client::zwp_text_input_manager_v3::ZwpTextInputManagerV3;
 use sctk::reexports::protocols::staging::xdg_activation::v1::client::xdg_activation_v1::XdgActivationV1;
+use sctk::reexports::protocols::viewporter::client::wp_viewporter::WpViewporter;
 
 use sctk::environment::{Environment, SimpleGlobal};
 use sctk::output::{OutputHandler, OutputHandling, OutputInfo, OutputStatusListener};
@@ -21,6 +22,8 @@ use sctk::seat::{SeatData, SeatHandler, SeatHandling, SeatListener};
 use sctk::shell::{Shell, ShellHandler, ShellHandling};
 use sctk::shm::ShmHandler;
 
+use crate::platform_impl::wayland::protocols::wp_fractional_scale_manager_v1::WpFractionalScaleManagerV1;
+
 /// Set of extra features that are supported by the compositor.
 #[derive(Debug, Clone, Copy)]
 pub struct WindowingFeatures {
@@ -61,6 +64,8 @@ sctk::environment!(WinitEnv,
         ZwpPointerConstraintsV1 => pointer_constraints,
         ZwpTextInputManagerV3 => text_input_manager,
         XdgActivationV1 => xdg_activation,
+        WpFractionalScaleManagerV1 => fractional_scale_manager,
+        WpViewporter => viewporter,
     ],
     multis = [
         WlSeat => seats,
@@ -91,6 +96,10 @@ pub struct WinitEnv {
     decoration_manager: SimpleGlobal<ZxdgDecorationManagerV1>,
 
     xdg_activation: SimpleGlobal<XdgActivationV1>,
+
+    fractional_scale_manager: SimpleGlobal<WpFractionalScaleManagerV1>,
+
+    viewporter: SimpleGlobal<WpViewporter>,
 }
 
 impl WinitEnv {
@@ -125,6 +134,12 @@ impl WinitEnv {
         // Surface activation.
         let xdg_activation = SimpleGlobal::new();
 
+        // Fractional surface scaling.
+        let fractional_scale_manager = SimpleGlobal::new();
+
+        // Surface resizing (used for fractional scaling).
+        let viewporter = SimpleGlobal::new();
+
         Self {
             seats,
             outputs,
@@ -137,6 +152,8 @@ impl WinitEnv {
             pointer_constraints,
             text_input_manager,
             xdg_activation,
+            fractional_scale_manager,
+            viewporter,
         }
     }
 }

src/platform_impl/linux/wayland/event_loop/mod.rs

@@ -374,10 +374,11 @@ impl<T: 'static> EventLoop<T> {
             });
 
             for (window_id, window_compositor_update) in window_compositor_updates.iter_mut() {
-                if let Some(scale_factor) = window_compositor_update.scale_factor.map(|f| f as f64)
-                {
+                if let Some(scale_factor) = window_compositor_update.scale_factor {
                     let mut physical_size = self.with_state(|state| {
                         let window_handle = state.window_map.get(window_id).unwrap();
+                        *window_handle.scale_factor.lock().unwrap() = scale_factor;
+
                         let mut size = window_handle.size.lock().unwrap();
 
                         // Update the new logical size if it was changed.
@@ -409,6 +410,15 @@ impl<T: 'static> EventLoop<T> {
                 if let Some(size) = window_compositor_update.size.take() {
                     let physical_size = self.with_state(|state| {
                         let window_handle = state.window_map.get_mut(window_id).unwrap();
+
+                        if let Some(fs_state) = window_handle.fractional_scaling_state.as_ref() {
+                            // If we have a viewport then we support fractional scaling. As per the
+                            // protocol, we have to set the viewport size of the size prior scaling.
+                            fs_state
+                                .viewport
+                                .set_destination(size.width as _, size.height as _);
+                        }
+
                         let mut window_size = window_handle.size.lock().unwrap();
 
                         // Always issue resize event on scale factor change.
@@ -419,9 +429,8 @@ impl<T: 'static> EventLoop<T> {
                             None
                         } else {
                             *window_size = size;
-                            let scale_factor =
-                                sctk::get_surface_scale_factor(window_handle.window.surface());
-                            let physical_size = size.to_physical(scale_factor as f64);
+                            let scale_factor = window_handle.scale_factor();
+                            let physical_size = size.to_physical(scale_factor);
                             Some(physical_size)
                         };
 

src/platform_impl/linux/wayland/mod.rs

@@ -10,6 +10,7 @@ pub use window::Window;
 mod env;
 mod event_loop;
 mod output;
+mod protocols;
 mod seat;
 mod window;
 

src/platform_impl/linux/wayland/protocols.rs

@@ -0,0 +1,13 @@
+#![allow(dead_code, non_camel_case_types, unused_unsafe, unused_variables)]
+#![allow(non_upper_case_globals, non_snake_case, unused_imports)]
+#![allow(missing_docs, clippy::all)]
+
+use wayland_client::protocol::wl_surface;
+use wayland_client::sys;
+use wayland_client::{AnonymousObject, Attached, Main, Proxy, ProxyMap};
+use wayland_commons::map::{Object, ObjectMetadata};
+use wayland_commons::smallvec;
+use wayland_commons::wire::{Argument, ArgumentType, Message, MessageDesc};
+use wayland_commons::{Interface, MessageGroup};
+
+include!(concat!(env!("OUT_DIR"), "/fractional_scale_v1.rs"));

src/platform_impl/linux/wayland/seat/pointer/handlers.rs

@@ -53,7 +53,7 @@ pub(super) fn handle_pointer(
                 None => return,
             };
 
-            let scale_factor = sctk::get_surface_scale_factor(&surface) as f64;
+            let scale_factor = window_handle.scale_factor();
             pointer_data.surface = Some(surface);
 
             // Notify window that pointer entered the surface.
@@ -133,8 +133,12 @@ pub(super) fn handle_pointer(
             };
 
             let window_id = wayland::make_wid(surface);
+            let window_handle = match winit_state.window_map.get(&window_id) {
+                Some(w) => w,
+                _ => return,
+            };
 
-            let scale_factor = sctk::get_surface_scale_factor(surface) as f64;
+            let scale_factor = window_handle.scale_factor();
             let position = LogicalPosition::new(surface_x, surface_y).to_physical(scale_factor);
 
             event_sink.push_window_event(
@@ -192,6 +196,10 @@ pub(super) fn handle_pointer(
             };
 
             let window_id = wayland::make_wid(surface);
+            let window_handle = match winit_state.window_map.get(&window_id) {
+                Some(w) => w,
+                _ => return,
+            };
 
             if pointer.as_ref().version() < 5 {
                 let (mut x, mut y) = (0.0, 0.0);
@@ -204,7 +212,7 @@ pub(super) fn handle_pointer(
                     _ => unreachable!(),
                 }
 
-                let scale_factor = sctk::get_surface_scale_factor(surface) as f64;
+                let scale_factor = window_handle.scale_factor();
                 let delta = LogicalPosition::new(x as f64, y as f64).to_physical(scale_factor);
 
                 event_sink.push_window_event(
@@ -268,6 +276,10 @@ pub(super) fn handle_pointer(
                 None => return,
             };
             let window_id = wayland::make_wid(surface);
+            let window_handle = match winit_state.window_map.get(&window_id) {
+                Some(w) => w,
+                _ => return,
+            };
 
             let window_event = if let Some((x, y)) = axis_discrete_buffer {
                 WindowEvent::MouseWheel {
@@ -279,7 +291,7 @@ pub(super) fn handle_pointer(
                     modifiers: *pointer_data.modifiers_state.borrow(),
                 }
             } else if let Some((x, y)) = axis_buffer {
-                let scale_factor = sctk::get_surface_scale_factor(surface) as f64;
+                let scale_factor = window_handle.scale_factor();
                 let delta = LogicalPosition::new(x, y).to_physical(scale_factor);
 
                 WindowEvent::MouseWheel {

src/platform_impl/linux/wayland/seat/touch/handlers.rs

@@ -24,11 +24,12 @@ pub(super) fn handle_touch(
             surface, id, x, y, ..
         } => {
             let window_id = wayland::make_wid(&surface);
-            if !winit_state.window_map.contains_key(&window_id) {
-                return;
-            }
+            let window_handle = match winit_state.window_map.get(&window_id) {
+                Some(w) => w,
+                _ => return,
+            };
 
-            let scale_factor = sctk::get_surface_scale_factor(&surface) as f64;
+            let scale_factor = window_handle.scale_factor();
             let position = LogicalPosition::new(x, y);
 
             event_sink.push_window_event(
@@ -60,7 +61,12 @@ pub(super) fn handle_touch(
                 None => return,
             };
 
-            let scale_factor = sctk::get_surface_scale_factor(&touch_point.surface) as f64;
+            let window_id = wayland::make_wid(&touch_point.surface);
+            let window_handle = match winit_state.window_map.get(&window_id) {
+                Some(w) => w,
+                _ => return,
+            };
+            let scale_factor = window_handle.scale_factor();
             let location = touch_point.position.to_physical(scale_factor);
             let window_id = wayland::make_wid(&touch_point.surface);
 
@@ -82,10 +88,15 @@ pub(super) fn handle_touch(
                 Some(touch_point) => touch_point,
                 None => return,
             };
+            let window_id = wayland::make_wid(&touch_point.surface);
+            let window_handle = match winit_state.window_map.get(&window_id) {
+                Some(w) => w,
+                _ => return,
+            };
 
             touch_point.position = LogicalPosition::new(x, y);
 
-            let scale_factor = sctk::get_surface_scale_factor(&touch_point.surface) as f64;
+            let scale_factor = window_handle.scale_factor();
             let location = touch_point.position.to_physical(scale_factor);
             let window_id = wayland::make_wid(&touch_point.surface);
 
@@ -105,9 +116,14 @@ pub(super) fn handle_touch(
         TouchEvent::Frame => (),
         TouchEvent::Cancel => {
             for touch_point in inner.touch_points.drain(..) {
-                let scale_factor = sctk::get_surface_scale_factor(&touch_point.surface) as f64;
-                let location = touch_point.position.to_physical(scale_factor);
                 let window_id = wayland::make_wid(&touch_point.surface);
+                let window_handle = match winit_state.window_map.get(&window_id) {
+                    Some(w) => w,
+                    _ => return,
+                };
+
+                let scale_factor = window_handle.scale_factor();
+                let location = touch_point.position.to_physical(scale_factor);
 
                 event_sink.push_window_event(
                     WindowEvent::Touch(crate::event::Touch {

src/platform_impl/linux/wayland/window/mod.rs

@@ -11,12 +11,14 @@ use raw_window_handle::{
     RawDisplayHandle, RawWindowHandle, WaylandDisplayHandle, WaylandWindowHandle,
 };
 use sctk::window::Decorations;
+use wayland_protocols::viewporter::client::wp_viewporter::WpViewporter;
 
 use crate::dpi::{LogicalSize, PhysicalPosition, PhysicalSize, Position, Size};
 use crate::error::{ExternalError, NotSupportedError, OsError as RootOsError};
 use crate::platform_impl::{
-    Fullscreen, MonitorHandle as PlatformMonitorHandle, OsError,
-    PlatformSpecificWindowBuilderAttributes as PlatformAttributes,
+    wayland::protocols::wp_fractional_scale_manager_v1::WpFractionalScaleManagerV1,
+    wayland::protocols::wp_fractional_scale_v1, Fullscreen, MonitorHandle as PlatformMonitorHandle,
+    OsError, PlatformSpecificWindowBuilderAttributes as PlatformAttributes,
 };
 use crate::window::{
     CursorGrabMode, CursorIcon, ResizeDirection, Theme, UserAttentionType, WindowAttributes,
@@ -30,7 +32,9 @@ use super::{EventLoopWindowTarget, WindowId};
 
 pub mod shim;
 
-use shim::{WindowCompositorUpdate, WindowHandle, WindowRequest, WindowUserRequest};
+use shim::{
+    FractionalScalingState, WindowCompositorUpdate, WindowHandle, WindowRequest, WindowUserRequest,
+};
 
 #[cfg(feature = "sctk-adwaita")]
 pub type WinitFrame = sctk_adwaita::AdwaitaFrame;
@@ -50,6 +54,9 @@ pub struct Window {
     /// The underlying wl_surface.
     surface: WlSurface,
 
+    /// The scale factor.
+    scale_factor: Arc<Mutex<f64>>,
+
     /// The current window size.
     size: Arc<Mutex<LogicalSize<u32>>>,
 
@@ -90,33 +97,50 @@ impl Window {
         attributes: WindowAttributes,
         platform_attributes: PlatformAttributes,
     ) -> Result<Self, RootOsError> {
-        let surface = event_loop_window_target
+        let viewporter = event_loop_window_target.env.get_global::<WpViewporter>();
+        let fractional_scale_manager = event_loop_window_target
             .env
-            .create_surface_with_scale_callback(move |scale, surface, mut dispatch_data| {
-                let winit_state = dispatch_data.get::<WinitState>().unwrap();
+            .get_global::<WpFractionalScaleManagerV1>();
+
+        // Create surface and register callback for the scale factor changes.
+        let mut scale_factor = 1.;
+        let (surface, fractional_scaling_state) =
+            if let (Some(viewporter), Some(fractional_scale_manager)) =
+                (viewporter, fractional_scale_manager)
+            {
+                let surface = event_loop_window_target.env.create_surface().detach();
+                let fractional_scale = fractional_scale_manager.get_fractional_scale(&surface);
 
-                // Get the window that received the event.
                 let window_id = super::make_wid(&surface);
-                let mut window_compositor_update = winit_state
-                    .window_compositor_updates
-                    .get_mut(&window_id)
-                    .unwrap();
+                fractional_scale.quick_assign(move |_, event, mut dispatch_data| {
+                    let wp_fractional_scale_v1::Event::PreferredScale { scale } = event;
+                    let winit_state = dispatch_data.get::<WinitState>().unwrap();
+                    apply_scale(window_id, scale as f64 / 120., winit_state);
+                });
 
-                // Mark that we need a frame refresh on the DPI change.
-                winit_state
-                    .window_user_requests
-                    .get_mut(&window_id)
-                    .unwrap()
-                    .refresh_frame = true;
+                let fractional_scale = fractional_scale.detach();
+                let viewport = viewporter.get_viewport(&surface).detach();
+                let fractional_scaling_state =
+                    FractionalScalingState::new(viewport, fractional_scale);
 
-                // Set pending scale factor.
-                window_compositor_update.scale_factor = Some(scale);
+                (surface, Some(fractional_scaling_state))
+            } else {
+                let surface = event_loop_window_target
+                    .env
+                    .create_surface_with_scale_callback(move |scale, surface, mut dispatch_data| {
+                        let winit_state = dispatch_data.get::<WinitState>().unwrap();
 
-                surface.set_buffer_scale(scale);
-            })
-            .detach();
+                        // Get the window that received the event.
+                        let window_id = super::make_wid(&surface);
+                        apply_scale(window_id, scale as f64, winit_state);
+                        surface.set_buffer_scale(scale);
+                    })
+                    .detach();
 
-        let scale_factor = sctk::get_surface_scale_factor(&surface);
+                scale_factor = sctk::get_surface_scale_factor(&surface) as _;
+
+                (surface, None)
+            };
 
         let window_id = super::make_wid(&surface);
         let maximized = Arc::new(AtomicBool::new(false));
@@ -126,7 +150,7 @@ impl Window {
 
         let (width, height) = attributes
             .inner_size
-            .map(|size| size.to_logical::<f64>(scale_factor as f64).into())
+            .map(|size| size.to_logical::<f64>(scale_factor).into())
             .unwrap_or((800, 600));
 
         let theme_manager = event_loop_window_target.theme_manager.clone();
@@ -194,13 +218,13 @@ impl Window {
         // Min dimensions.
         let min_size = attributes
             .min_inner_size
-            .map(|size| size.to_logical::<f64>(scale_factor as f64).into());
+            .map(|size| size.to_logical::<f64>(scale_factor).into());
         window.set_min_size(min_size);
 
         // Max dimensions.
         let max_size = attributes
             .max_inner_size
-            .map(|size| size.to_logical::<f64>(scale_factor as f64).into());
+            .map(|size| size.to_logical::<f64>(scale_factor).into());
         window.set_max_size(max_size);
 
         // Set Wayland specific window attributes.
@@ -263,6 +287,8 @@ impl Window {
             window,
             size.clone(),
             has_focus.clone(),
+            fractional_scaling_state,
+            scale_factor,
             window_requests.clone(),
         );
 
@@ -324,6 +350,7 @@ impl Window {
             decorated: AtomicBool::new(attributes.decorations),
             cursor_grab_mode: Mutex::new(CursorGrabMode::None),
             has_focus,
+            scale_factor: window_handle.scale_factor.clone(),
         };
 
         Ok(window)
@@ -372,10 +399,7 @@ impl Window {
     }
 
     pub fn inner_size(&self) -> PhysicalSize<u32> {
-        self.size
-            .lock()
-            .unwrap()
-            .to_physical(self.scale_factor() as f64)
+        self.size.lock().unwrap().to_physical(self.scale_factor())
     }
 
     #[inline]
@@ -385,15 +409,12 @@ impl Window {
 
     #[inline]
     pub fn outer_size(&self) -> PhysicalSize<u32> {
-        self.size
-            .lock()
-            .unwrap()
-            .to_physical(self.scale_factor() as f64)
+        self.size.lock().unwrap().to_physical(self.scale_factor())
     }
 
     #[inline]
     pub fn set_inner_size(&self, size: Size) {
-        let scale_factor = self.scale_factor() as f64;
+        let scale_factor = self.scale_factor();
 
         let size = size.to_logical::<u32>(scale_factor);
         *self.size.lock().unwrap() = size;
@@ -403,7 +424,7 @@ impl Window {
 
     #[inline]
     pub fn set_min_inner_size(&self, dimensions: Option<Size>) {
-        let scale_factor = self.scale_factor() as f64;
+        let scale_factor = self.scale_factor();
         let size = dimensions.map(|size| size.to_logical::<u32>(scale_factor));
 
         self.send_request(WindowRequest::MinSize(size));
@@ -411,7 +432,7 @@ impl Window {
 
     #[inline]
     pub fn set_max_inner_size(&self, dimensions: Option<Size>) {
-        let scale_factor = self.scale_factor() as f64;
+        let scale_factor = self.scale_factor();
         let size = dimensions.map(|size| size.to_logical::<u32>(scale_factor));
 
         self.send_request(WindowRequest::MaxSize(size));
@@ -447,10 +468,8 @@ impl Window {
     }
 
     #[inline]
-    pub fn scale_factor(&self) -> u32 {
-        // The scale factor from `get_surface_scale_factor` is always greater than zero, so
-        // u32 conversion is safe.
-        sctk::get_surface_scale_factor(&self.surface) as u32
+    pub fn scale_factor(&self) -> f64 {
+        *self.scale_factor.lock().unwrap()
     }
 
     #[inline]
@@ -561,7 +580,7 @@ impl Window {
             ))));
         }
 
-        let scale_factor = self.scale_factor() as f64;
+        let scale_factor = self.scale_factor();
         let position = position.to_logical(scale_factor);
         self.send_request(WindowRequest::SetLockedCursorPosition(position));
 
@@ -589,7 +608,7 @@ impl Window {
 
     #[inline]
     pub fn set_ime_position(&self, position: Position) {
-        let scale_factor = self.scale_factor() as f64;
+        let scale_factor = self.scale_factor();
         let position = position.to_logical(scale_factor);
         self.send_request(WindowRequest::ImePosition(position));
     }
@@ -700,3 +719,20 @@ impl TryFrom<&str> for Theme {
         }
     }
 }
+
+/// Set pending scale for the provided `window_id`.
+fn apply_scale(window_id: WindowId, scale: f64, winit_state: &mut WinitState) {
+    // Set pending scale factor.
+    winit_state
+        .window_compositor_updates
+        .get_mut(&window_id)
+        .unwrap()
+        .scale_factor = Some(scale);
+
+    // Mark that we need a frame refresh on the DPI change.
+    winit_state
+        .window_user_requests
+        .get_mut(&window_id)
+        .unwrap()
+        .refresh_frame = true;
+}

src/platform_impl/linux/wayland/window/shim.rs

@@ -11,6 +11,7 @@ use sctk::reexports::protocols::staging::xdg_activation::v1::client::xdg_activat
 
 use sctk::environment::Environment;
 use sctk::window::{Decorations, Window};
+use wayland_protocols::viewporter::client::wp_viewport::WpViewport;
 
 use crate::dpi::{LogicalPosition, LogicalSize};
 
@@ -18,6 +19,7 @@ use crate::event::{Ime, WindowEvent};
 use crate::platform_impl::wayland;
 use crate::platform_impl::wayland::env::WinitEnv;
 use crate::platform_impl::wayland::event_loop::{EventSink, WinitState};
+use crate::platform_impl::wayland::protocols::wp_fractional_scale_v1::WpFractionalScaleV1;
 use crate::platform_impl::wayland::seat::pointer::WinitPointer;
 use crate::platform_impl::wayland::seat::text_input::TextInputHandler;
 use crate::platform_impl::wayland::WindowId;
@@ -109,7 +111,7 @@ pub struct WindowCompositorUpdate {
     pub size: Option<LogicalSize<u32>>,
 
     /// New scale factor.
-    pub scale_factor: Option<i32>,
+    pub scale_factor: Option<f64>,
 
     /// Close the window.
     pub close_window: bool,
@@ -143,6 +145,12 @@ pub struct WindowHandle {
     /// An actual window.
     pub window: ManuallyDrop<Window<WinitFrame>>,
 
+    /// The state of the fractional scaling handlers for the window.
+    pub fractional_scaling_state: Option<FractionalScalingState>,
+
+    /// The scale factor of the window.
+    pub scale_factor: Arc<Mutex<f64>>,
+
     /// The current size of the window.
     pub size: Arc<Mutex<LogicalSize<u32>>>,
 
@@ -192,6 +200,8 @@ impl WindowHandle {
         window: Window<WinitFrame>,
         size: Arc<Mutex<LogicalSize<u32>>>,
         has_focus: Arc<AtomicBool>,
+        fractional_scaling_state: Option<FractionalScalingState>,
+        scale_factor: f64,
         pending_window_requests: Arc<Mutex<Vec<WindowRequest>>>,
     ) -> Self {
         let xdg_activation = env.get_global::<XdgActivationV1>();
@@ -201,6 +211,8 @@ impl WindowHandle {
 
         Self {
             window: ManuallyDrop::new(window),
+            fractional_scaling_state,
+            scale_factor: Arc::new(Mutex::new(scale_factor)),
             size,
             pending_window_requests,
             cursor_icon: Cell::new(CursorIcon::Default),
@@ -218,6 +230,10 @@ impl WindowHandle {
         }
     }
 
+    pub fn scale_factor(&self) -> f64 {
+        *self.scale_factor.lock().unwrap()
+    }
+
     pub fn set_cursor_grab(&self, mode: CursorGrabMode) {
         // The new requested state matches the current confine status, return.
         let old_mode = self.cursor_grab_mode.replace(mode);
@@ -587,6 +603,9 @@ pub fn handle_window_requests(winit_state: &mut WinitState) {
 
 impl Drop for WindowHandle {
     fn drop(&mut self) {
+        // Drop the fractional scaling before the surface.
+        let _ = self.fractional_scaling_state.take();
+
         unsafe {
             let surface = self.window.surface().clone();
             // The window must be destroyed before wl_surface.
@@ -595,3 +614,28 @@ impl Drop for WindowHandle {
         }
     }
 }
+
+/// Fractional scaling objects.
+pub struct FractionalScalingState {
+    /// The wp-viewport of the window.
+    pub viewport: WpViewport,
+
+    /// The wp-fractional-scale of the window surface.
+    pub fractional_scale: WpFractionalScaleV1,
+}
+
+impl FractionalScalingState {
+    pub fn new(viewport: WpViewport, fractional_scale: WpFractionalScaleV1) -> Self {
+        Self {
+            viewport,
+            fractional_scale,
+        }
+    }
+}
+
+impl Drop for FractionalScalingState {
+    fn drop(&mut self) {
+        self.viewport.destroy();
+        self.fractional_scale.destroy();
+    }
+}

src/window.rs

@@ -486,6 +486,7 @@ impl Window {
     /// ## Platform-specific
     ///
     /// - **X11:** This respects Xft.dpi, and can be overridden using the `WINIT_X11_SCALE_FACTOR` environment variable.
+    /// - **Wayland:** Uses the wp-fractional-scale protocol if available. Falls back to integer-scale factors otherwise.
     /// - **Android:** Always returns 1.0.
     /// - **iOS:** Can only be called on the main thread. Returns the underlying `UIView`'s
     ///   [`contentScaleFactor`].

wayland_protocols/fractional-scale-v1.xml

@@ -0,0 +1,102 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<protocol name="fractional_scale_v1">
+  <copyright>
+    Copyright © 2022 Kenny Levinsen
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice (including the next
+    paragraph) shall be included in all copies or substantial portions of the
+    Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+  </copyright>
+
+  <description summary="Protocol for requesting fractional surface scales">
+    This protocol allows a compositor to suggest for surfaces to render at
+    fractional scales.
+
+    A client can submit scaled content by utilizing wp_viewport. This is done by
+    creating a wp_viewport object for the surface and setting the destination
+    rectangle to the surface size before the scale factor is applied.
+
+    The buffer size is calculated by multiplying the surface size by the
+    intended scale.
+
+    The wl_surface buffer scale should remain set to 1.
+
+    If a surface has a surface-local size of 100 px by 50 px and wishes to
+    submit buffers with a scale of 1.5, then a buffer of 150px by 75 px should
+    be used and the wp_viewport destination rectangle should be 100 px by 50 px.
+
+    For toplevel surfaces, the size is rounded halfway away from zero. The
+    rounding algorithm for subsurface position and size is not defined.
+  </description>
+
+  <interface name="wp_fractional_scale_manager_v1" version="1">
+    <description summary="fractional surface scale information">
+      A global interface for requesting surfaces to use fractional scales.
+    </description>
+
+    <request name="destroy" type="destructor">
+      <description summary="unbind the fractional surface scale interface">
+        Informs the server that the client will not be using this protocol
+        object anymore. This does not affect any other objects,
+        wp_fractional_scale_v1 objects included.
+      </description>
+    </request>
+
+    <enum name="error">
+      <entry name="fractional_scale_exists" value="0"
+        summary="the surface already has a fractional_scale object associated"/>
+    </enum>
+
+    <request name="get_fractional_scale">
+      <description summary="extend surface interface for scale information">
+        Create an add-on object for the the wl_surface to let the compositor
+        request fractional scales. If the given wl_surface already has a
+        wp_fractional_scale_v1 object associated, the fractional_scale_exists
+        protocol error is raised.
+      </description>
+      <arg name="id" type="new_id" interface="wp_fractional_scale_v1"
+           summary="the new surface scale info interface id"/>
+      <arg name="surface" type="object" interface="wl_surface"
+           summary="the surface"/>
+    </request>
+  </interface>
+
+  <interface name="wp_fractional_scale_v1" version="1">
+    <description summary="fractional scale interface to a wl_surface">
+      An additional interface to a wl_surface object which allows the compositor
+      to inform the client of the preferred scale.
+    </description>
+
+    <request name="destroy" type="destructor">
+      <description summary="remove surface scale information for surface">
+        Destroy the fractional scale object. When this object is destroyed,
+        preferred_scale events will no longer be sent.
+      </description>
+    </request>
+
+    <event name="preferred_scale">
+      <description summary="notify of new preferred scale">
+        Notification of a new preferred scale for this surface that the
+        compositor suggests that the client should use.
+
+        The sent scale is the numerator of a fraction with a denominator of 120.
+      </description>
+      <arg name="scale" type="uint" summary="the new preferred scale"/>
+    </event>
+  </interface>
+</protocol>