Add web support for `zoom_factor` (#4260)

Before, when setting the `zoom_factor`, the website was already
enlarged, but the zoom was ignored when calculating the logical window
size and mouse position, causing an offset between the actual cursor and
selected elements. That is addressed here

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/eframe/src/web/app_runner.rs

@@ -60,7 +60,9 @@ impl AppRunner {
         super::storage::load_memory(&egui_ctx);
 
         egui_ctx.options_mut(|o| {
-            // On web, the browser controls the zoom factor:
+            // On web by default egui follows the zoom factor of the browser,
+            // and lets the browser handle the zoom shortscuts.
+            // A user can still zoom egui separately by calling [`egui::Context::set_zoom_factor`].
             o.zoom_with_keyboard = false;
             o.zoom_factor = 1.0;
         });
@@ -183,7 +185,7 @@ 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());
+        let canvas_size = super::canvas_size_in_points(self.canvas(), self.egui_ctx());
         let raw_input = self.input.new_frame(canvas_size);
 
         let full_output = self.egui_ctx.run(raw_input, |egui_ctx| {

crates/eframe/src/web/events.rs

@@ -297,7 +297,7 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
             let modifiers = modifiers_from_mouse_event(&event);
             runner.input.raw.modifiers = modifiers;
             if let Some(button) = button_from_mouse_event(&event) {
-                let pos = pos_from_mouse_event(runner.canvas(), &event);
+                let pos = pos_from_mouse_event(runner.canvas(), &event, runner.egui_ctx());
                 let modifiers = runner.input.raw.modifiers;
                 runner.input.raw.events.push(egui::Event::PointerButton {
                     pos,
@@ -324,7 +324,7 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
         |event: web_sys::MouseEvent, runner| {
             let modifiers = modifiers_from_mouse_event(&event);
             runner.input.raw.modifiers = modifiers;
-            let pos = pos_from_mouse_event(runner.canvas(), &event);
+            let pos = pos_from_mouse_event(runner.canvas(), &event, runner.egui_ctx());
             runner.input.raw.events.push(egui::Event::PointerMoved(pos));
             runner.needs_repaint.repaint_asap();
             event.stop_propagation();
@@ -336,7 +336,7 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
         let modifiers = modifiers_from_mouse_event(&event);
         runner.input.raw.modifiers = modifiers;
         if let Some(button) = button_from_mouse_event(&event) {
-            let pos = pos_from_mouse_event(runner.canvas(), &event);
+            let pos = pos_from_mouse_event(runner.canvas(), &event, runner.egui_ctx());
             let modifiers = runner.input.raw.modifiers;
             runner.input.raw.events.push(egui::Event::PointerButton {
                 pos,
@@ -374,7 +374,12 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
         "touchstart",
         |event: web_sys::TouchEvent, runner| {
             let mut latest_touch_pos_id = runner.input.latest_touch_pos_id;
-            let pos = pos_from_touch_event(runner.canvas(), &event, &mut latest_touch_pos_id);
+            let pos = pos_from_touch_event(
+                runner.canvas(),
+                &event,
+                &mut latest_touch_pos_id,
+                runner.egui_ctx(),
+            );
             runner.input.latest_touch_pos_id = latest_touch_pos_id;
             runner.input.latest_touch_pos = Some(pos);
             let modifiers = runner.input.raw.modifiers;
@@ -397,7 +402,12 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
         "touchmove",
         |event: web_sys::TouchEvent, runner| {
             let mut latest_touch_pos_id = runner.input.latest_touch_pos_id;
-            let pos = pos_from_touch_event(runner.canvas(), &event, &mut latest_touch_pos_id);
+            let pos = pos_from_touch_event(
+                runner.canvas(),
+                &event,
+                &mut latest_touch_pos_id,
+                runner.egui_ctx(),
+            );
             runner.input.latest_touch_pos_id = latest_touch_pos_id;
             runner.input.latest_touch_pos = Some(pos);
             runner.input.raw.events.push(egui::Event::PointerMoved(pos));
@@ -460,7 +470,9 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
         });
 
         let scroll_multiplier = match unit {
-            egui::MouseWheelUnit::Page => canvas_size_in_points(runner.canvas()).y,
+            egui::MouseWheelUnit::Page => {
+                canvas_size_in_points(runner.canvas(), runner.egui_ctx()).y
+            }
             egui::MouseWheelUnit::Line => {
                 #[allow(clippy::let_and_return)]
                 let points_per_scroll_line = 8.0; // Note that this is intentionally different from what we use in winit.

crates/eframe/src/web/input.rs

@@ -3,11 +3,13 @@ use super::{canvas_origin, AppRunner};
 pub fn pos_from_mouse_event(
     canvas: &web_sys::HtmlCanvasElement,
     event: &web_sys::MouseEvent,
+    ctx: &egui::Context,
 ) -> egui::Pos2 {
     let rect = canvas.get_bounding_client_rect();
+    let zoom_factor = ctx.zoom_factor();
     egui::Pos2 {
-        x: event.client_x() as f32 - rect.left() as f32,
-        y: event.client_y() as f32 - rect.top() as f32,
+        x: (event.client_x() as f32 - rect.left() as f32) / zoom_factor,
+        y: (event.client_y() as f32 - rect.top() as f32) / zoom_factor,
     }
 }
 
@@ -32,6 +34,7 @@ pub fn pos_from_touch_event(
     canvas: &web_sys::HtmlCanvasElement,
     event: &web_sys::TouchEvent,
     touch_id_for_pos: &mut Option<egui::TouchId>,
+    egui_ctx: &egui::Context,
 ) -> egui::Pos2 {
     let touch_for_pos = if let Some(touch_id_for_pos) = touch_id_for_pos {
         // search for the touch we previously used for the position
@@ -49,14 +52,19 @@ pub fn pos_from_touch_event(
         .or_else(|| event.touches().get(0))
         .map_or(Default::default(), |touch| {
             *touch_id_for_pos = Some(egui::TouchId::from(touch.identifier()));
-            pos_from_touch(canvas_origin(canvas), &touch)
+            pos_from_touch(canvas_origin(canvas), &touch, egui_ctx)
         })
 }
 
-fn pos_from_touch(canvas_origin: egui::Pos2, touch: &web_sys::Touch) -> egui::Pos2 {
+fn pos_from_touch(
+    canvas_origin: egui::Pos2,
+    touch: &web_sys::Touch,
+    egui_ctx: &egui::Context,
+) -> egui::Pos2 {
+    let zoom_factor = egui_ctx.zoom_factor();
     egui::Pos2 {
-        x: touch.page_x() as f32 - canvas_origin.x,
-        y: touch.page_y() as f32 - canvas_origin.y,
+        x: (touch.page_x() as f32 - canvas_origin.x) / zoom_factor,
+        y: (touch.page_y() as f32 - canvas_origin.y) / zoom_factor,
     }
 }
 
@@ -68,7 +76,7 @@ pub fn push_touches(runner: &mut AppRunner, phase: egui::TouchPhase, event: &web
                 device_id: egui::TouchDeviceId(0),
                 id: egui::TouchId::from(touch.identifier()),
                 phase,
-                pos: pos_from_touch(canvas_origin, &touch),
+                pos: pos_from_touch(canvas_origin, &touch, runner.egui_ctx()),
                 force: Some(touch.force()),
             });
         }

crates/eframe/src/web/mod.rs

@@ -116,8 +116,8 @@ fn canvas_origin(canvas: &web_sys::HtmlCanvasElement) -> egui::Pos2 {
     egui::pos2(rect.left() as f32, rect.top() as f32)
 }
 
-fn canvas_size_in_points(canvas: &web_sys::HtmlCanvasElement) -> egui::Vec2 {
-    let pixels_per_point = native_pixels_per_point();
+fn canvas_size_in_points(canvas: &web_sys::HtmlCanvasElement, ctx: &egui::Context) -> egui::Vec2 {
+    let pixels_per_point = ctx.pixels_per_point();
     egui::vec2(
         canvas.width() as f32 / pixels_per_point,
         canvas.height() as f32 / pixels_per_point,

crates/egui/src/memory.rs

@@ -185,6 +185,12 @@ pub struct Options {
     /// presses Cmd+Plus, Cmd+Minus or Cmd+0, just like in a browser.
     ///
     /// This is `true` by default.
+    ///
+    /// On the web-backend of `eframe` this is set to false by default,
+    /// so that the zoom shortcuts are handled exclusively by the browser,
+    /// which will change the `native_pixels_per_point` (`devicePixelRatio`).
+    /// You can still zoom egui independently by calling [`crate::Context::set_zoom_factor`],
+    /// which will be applied on top of the browsers global zoom.
     #[cfg_attr(feature = "serde", serde(skip))]
     pub zoom_with_keyboard: bool,