Use ResizeObserver instead of resize event

crates/eframe/Cargo.toml

@@ -236,7 +236,14 @@ web-sys = { workspace = true, features = [
   "MediaQueryListEvent",
   "MouseEvent",
   "Navigator",
+  "Node",
+  "NodeList",
   "Performance",
+  "ResizeObserver",
+  "ResizeObserverEntry",
+  "ResizeObserverBoxOptions",
+  "ResizeObserverOptions",
+  "ResizeObserverSize",
   "Storage",
   "Touch",
   "TouchEvent",

crates/eframe/src/epi.rs

@@ -455,11 +455,6 @@ pub struct WebOptions {
     /// Configures wgpu instance/device/adapter/surface creation and renderloop.
     #[cfg(feature = "wgpu")]
     pub wgpu_options: egui_wgpu::WgpuConfiguration,
-
-    /// The size limit of the web app canvas.
-    ///
-    /// By default the max size is [`egui::Vec2::INFINITY`], i.e. unlimited.
-    pub max_size_points: egui::Vec2,
 }
 
 #[cfg(target_arch = "wasm32")]
@@ -475,8 +470,6 @@ impl Default for WebOptions {
 
             #[cfg(feature = "wgpu")]
             wgpu_options: egui_wgpu::WgpuConfiguration::default(),
-
-            max_size_points: egui::Vec2::INFINITY,
         }
     }
 }

crates/eframe/src/web/app_runner.rs

@@ -5,6 +5,7 @@ use crate::{epi, App};
 use super::{now_sec, web_painter::WebPainter, NeedRepaint};
 
 pub struct AppRunner {
+    #[allow(dead_code)]
     web_options: crate::WebOptions,
     pub(crate) frame: epi::Frame,
     egui_ctx: egui::Context,
@@ -184,7 +185,6 @@ impl AppRunner {
     ///
     /// The result can be painted later with a call to [`Self::run_and_paint`] or [`Self::paint`].
     pub fn logic(&mut self) {
-        super::resize_canvas_to_screen_size(self.canvas(), self.web_options.max_size_points);
         let canvas_size = super::canvas_size_in_points(self.canvas(), self.egui_ctx());
         let raw_input = self.input.new_frame(canvas_size);
 

crates/eframe/src/web/events.rs

@@ -247,7 +247,8 @@ pub(crate) fn install_window_events(runner_ref: &WebRunner) -> Result<(), JsValu
         runner.save();
     })?;
 
-    for event_name in &["load", "pagehide", "pageshow", "resize"] {
+    // NOTE: resize is handled by `ResizeObserver` below
+    for event_name in &["load", "pagehide", "pageshow"] {
         runner_ref.add_event_listener(&window, event_name, move |_: web_sys::Event, runner| {
             // log::debug!("{event_name:?}");
             runner.needs_repaint.repaint_asap();
@@ -590,3 +591,79 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
 
     Ok(())
 }
+
+pub(crate) fn install_resize_observer(runner_ref: &WebRunner) -> Result<(), JsValue> {
+    let closure = Closure::wrap(Box::new({
+        let runner_ref = runner_ref.clone();
+        move |entries: js_sys::Array| {
+            // Only call the wrapped closure if the egui code has not panicked
+            if let Some(mut runner_lock) = runner_ref.try_lock() {
+                let canvas = runner_lock.canvas();
+                let (width, height) = match get_display_size(&entries) {
+                    Ok(v) => v,
+                    Err(err) => {
+                        log::error!("{}", super::string_from_js_value(&err));
+                        return;
+                    }
+                };
+                canvas.set_width(width);
+                canvas.set_height(height);
+
+                // force an immediate repaint
+                runner_lock.needs_repaint.repaint_asap();
+                paint_if_needed(&mut runner_lock);
+            }
+        }
+    }) as Box<dyn FnMut(js_sys::Array)>);
+
+    let observer = web_sys::ResizeObserver::new(closure.as_ref().unchecked_ref())?;
+    let mut options = web_sys::ResizeObserverOptions::new();
+    options.box_(web_sys::ResizeObserverBoxOptions::ContentBox);
+    if let Some(runner_lock) = runner_ref.try_lock() {
+        observer.observe_with_options(runner_lock.canvas(), &options);
+        drop(runner_lock);
+        runner_ref.set_resize_observer(observer, closure);
+    }
+
+    Ok(())
+}
+
+// Code ported to Rust from:
+// https://webglfundamentals.org/webgl/lessons/webgl-resizing-the-canvas.html
+fn get_display_size(resize_observer_entries: &js_sys::Array) -> Result<(u32, u32), JsValue> {
+    let width;
+    let height;
+    let mut dpr = web_sys::window().unwrap().device_pixel_ratio();
+
+    let entry: web_sys::ResizeObserverEntry = resize_observer_entries.at(0).dyn_into()?;
+    if JsValue::from_str("devicePixelContentBoxSize").js_in(entry.as_ref()) {
+        // NOTE: Only this path gives the correct answer for most browsers.
+        // Unfortunately this doesn't work perfectly everywhere.
+        let size: web_sys::ResizeObserverSize =
+            entry.device_pixel_content_box_size().at(0).dyn_into()?;
+        width = size.inline_size();
+        height = size.block_size();
+        dpr = 1.0; // no need to apply
+    } else if JsValue::from_str("contentBoxSize").js_in(entry.as_ref()) {
+        let content_box_size = entry.content_box_size();
+        let idx0 = content_box_size.at(0);
+        if !idx0.is_undefined() {
+            let size: web_sys::ResizeObserverSize = idx0.dyn_into()?;
+            width = size.inline_size();
+            height = size.block_size();
+        } else {
+            // legacy
+            let size = JsValue::clone(content_box_size.as_ref());
+            let size: web_sys::ResizeObserverSize = size.dyn_into()?;
+            width = size.inline_size();
+            height = size.block_size();
+        }
+    } else {
+        // legacy
+        let content_rect = entry.content_rect();
+        width = content_rect.width();
+        height = content_rect.height();
+    }
+
+    Ok(((width.round() * dpr) as u32, (height.round() * dpr) as u32))
+}

crates/eframe/src/web/mod.rs

@@ -40,7 +40,6 @@ pub(crate) type ActiveWebPainter = web_painter_wgpu::WebPainterWgpu;
 
 pub use backend::*;
 
-use egui::Vec2;
 use wasm_bindgen::prelude::*;
 use web_sys::MediaQueryList;
 
@@ -124,56 +123,6 @@ fn canvas_size_in_points(canvas: &web_sys::HtmlCanvasElement, ctx: &egui::Contex
     )
 }
 
-fn resize_canvas_to_screen_size(
-    canvas: &web_sys::HtmlCanvasElement,
-    max_size_points: egui::Vec2,
-) -> Option<()> {
-    let parent = canvas.parent_element()?;
-
-    // In this function we use "pixel" to mean physical pixel,
-    // and "point" to mean "logical CSS pixel".
-    let pixels_per_point = native_pixels_per_point();
-
-    // Prefer the client width and height so that if the parent
-    // element is resized that the egui canvas resizes appropriately.
-    let parent_size_points = Vec2 {
-        x: parent.client_width() as f32,
-        y: parent.client_height() as f32,
-    };
-
-    if parent_size_points.x <= 0.0 || parent_size_points.y <= 0.0 {
-        log::error!("The parent element of the egui canvas is {}x{}. Try adding `html, body {{ height: 100%; width: 100% }}` to your CSS!", parent_size_points.x, parent_size_points.y);
-    }
-
-    // We take great care here to ensure the rendered canvas aligns
-    // perfectly to the physical pixel grid, lest we get blurry text.
-    // At the time of writing, we get pixel perfection on Chromium and Firefox on Mac,
-    // but Desktop Safari will be blurry on most zoom levels.
-    // See https://github.com/emilk/egui/issues/4241 for more.
-
-    let canvas_size_pixels = pixels_per_point * parent_size_points.min(max_size_points);
-
-    // Make sure that the size is always an even number of pixels,
-    // otherwise, the page renders blurry on some platforms.
-    // See https://github.com/emilk/egui/issues/103
-    let canvas_size_pixels = (canvas_size_pixels / 2.0).round() * 2.0;
-
-    let canvas_size_points = canvas_size_pixels / pixels_per_point;
-
-    canvas
-        .style()
-        .set_property("width", &format!("{}px", canvas_size_points.x))
-        .ok()?;
-    canvas
-        .style()
-        .set_property("height", &format!("{}px", canvas_size_points.y))
-        .ok()?;
-    canvas.set_width(canvas_size_pixels.x as u32);
-    canvas.set_height(canvas_size_pixels.y as u32);
-
-    Some(())
-}
-
 // ----------------------------------------------------------------------------
 
 /// Set the cursor icon.

crates/eframe/src/web/web_runner.rs

@@ -28,8 +28,10 @@ pub struct WebRunner {
     /// the panic handler, since they aren't `Send`.
     events_to_unsubscribe: Rc<RefCell<Vec<EventToUnsubscribe>>>,
 
-    /// Used in `destroy` to cancel a pending frame.
+    /// Current animation frame in flight.
     request_animation_frame_id: Cell<Option<i32>>,
+
+    resize_observer: Rc<RefCell<Option<ResizeObserverContext>>>,
 }
 
 impl WebRunner {
@@ -48,6 +50,7 @@ impl WebRunner {
             runner: Rc::new(RefCell::new(None)),
             events_to_unsubscribe: Rc::new(RefCell::new(Default::default())),
             request_animation_frame_id: Cell::new(None),
+            resize_observer: Default::default(),
         }
     }
 
@@ -78,6 +81,8 @@ impl WebRunner {
                 events::install_color_scheme_change_event(self)?;
             }
 
+            events::install_resize_observer(self)?;
+
             self.request_animation_frame()?;
         }
 
@@ -109,6 +114,11 @@ impl WebRunner {
                 }
             }
         }
+
+        if let Some(context) = self.resize_observer.take() {
+            context.resize_observer.disconnect();
+            drop(context.closure);
+        }
     }
 
     /// Shut down eframe and clean up resources.
@@ -198,15 +208,35 @@ impl WebRunner {
             let runner_ref = self.clone();
             move || events::paint_and_schedule(&runner_ref)
         });
+
         let id = window.request_animation_frame(closure.as_ref().unchecked_ref())?;
         self.request_animation_frame_id.set(Some(id));
         closure.forget(); // We must forget it, or else the callback is canceled on drop
+
         Ok(())
     }
+
+    pub(crate) fn set_resize_observer(
+        &self,
+        resize_observer: web_sys::ResizeObserver,
+        closure: Closure<dyn FnMut(js_sys::Array)>,
+    ) {
+        self.resize_observer
+            .borrow_mut()
+            .replace(ResizeObserverContext {
+                resize_observer,
+                closure,
+            });
+    }
 }
 
 // ----------------------------------------------------------------------------
 
+struct ResizeObserverContext {
+    resize_observer: web_sys::ResizeObserver,
+    closure: Closure<dyn FnMut(js_sys::Array)>,
+}
+
 struct TargetEvent {
     target: web_sys::EventTarget,
     event_name: String,

web_demo/index.html

@@ -48,9 +48,10 @@
             margin-left: auto;
             display: block;
             position: absolute;
-            top: 0%;
-            left: 50%;
-            transform: translate(-50%, 0%);
+            top: 0;
+            left: 0;
+            width: 100%;
+            height: 100%;
         }
 
         .centered {