eframe web: Don't throw away frames on click/copy/cut (#3623)

* Follow-up to #3621 and
#3513

To work around a Safari limitation, we run the app logic in the event
handler of copy, cut, and mouse up and down.

Previously the output of that frame was discarded, but in this PR it is
now saved to be used in the next requestAnimationFrame.

The result is noticeable more distinct clicks on buttons (one more frame
of highlight)

Bonus: also fix auto-save of a sleeping web app

crates/eframe/src/web/app_runner.rs

@@ -1,5 +1,4 @@
 use egui::TexturesDelta;
-use wasm_bindgen::JsValue;
 
 use crate::{epi, App};
 
@@ -17,7 +16,10 @@ pub struct AppRunner {
     screen_reader: super::screen_reader::ScreenReader,
     pub(crate) text_cursor_pos: Option<egui::Pos2>,
     pub(crate) mutable_text_under_cursor: bool,
+
+    // Output for the last run:
     textures_delta: TexturesDelta,
+    clipped_primitives: Option<Vec<egui::ClippedPrimitive>>,
 }
 
 impl Drop for AppRunner {
@@ -115,6 +117,7 @@ impl AppRunner {
             text_cursor_pos: None,
             mutable_text_under_cursor: false,
             textures_delta: Default::default(),
+            clipped_primitives: None,
         };
 
         runner.input.raw.max_texture_side = Some(runner.painter.max_texture_side());
@@ -170,8 +173,26 @@ impl AppRunner {
         self.painter.destroy();
     }
 
-    /// Call [`Self::paint`] later to paint
-    pub fn logic(&mut self) -> Vec<egui::ClippedPrimitive> {
+    /// Runs the user code and paints the UI.
+    ///
+    /// If there is already an outstanding frame of output,
+    /// that is painted instead.
+    pub fn run_and_paint(&mut self) {
+        if self.clipped_primitives.is_none() {
+            // Run user code, and paint the results:
+            self.logic();
+            self.paint();
+        } else {
+            // We have already run the logic, e.g. in an on-click event,
+            // so let's only present the results:
+            self.paint();
+        }
+    }
+
+    /// Runs the logic, but doesn't paint the result.
+    ///
+    /// The result can be painted later with a call to [`Self::run_and_paint`] or [`Self::paint`].
+    pub fn logic(&mut self) {
         let frame_start = now_sec();
 
         super::resize_canvas_to_screen_size(self.canvas_id(), self.web_options.max_size_points);
@@ -203,25 +224,26 @@ impl AppRunner {
 
         self.handle_platform_output(platform_output);
         self.textures_delta.append(textures_delta);
-        let clipped_primitives = self.egui_ctx.tessellate(shapes, pixels_per_point);
+        self.clipped_primitives = Some(self.egui_ctx.tessellate(shapes, pixels_per_point));
 
         self.frame.info.cpu_usage = Some((now_sec() - frame_start) as f32);
-
-        clipped_primitives
     }
 
     /// Paint the results of the last call to [`Self::logic`].
-    pub fn paint(&mut self, clipped_primitives: &[egui::ClippedPrimitive]) -> Result<(), JsValue> {
+    pub fn paint(&mut self) {
         let textures_delta = std::mem::take(&mut self.textures_delta);
-
-        self.painter.paint_and_update_textures(
-            self.app.clear_color(&self.egui_ctx.style().visuals),
-            clipped_primitives,
-            self.egui_ctx.pixels_per_point(),
-            &textures_delta,
-        )?;
-
-        Ok(())
+        let clipped_primitives = std::mem::take(&mut self.clipped_primitives);
+
+        if let Some(clipped_primitives) = clipped_primitives {
+            if let Err(err) = self.painter.paint_and_update_textures(
+                self.app.clear_color(&self.egui_ctx.style().visuals),
+                &clipped_primitives,
+                self.egui_ctx.pixels_per_point(),
+                &textures_delta,
+            ) {
+                log::error!("Failed to paint: {}", super::string_from_js_value(&err));
+            }
+        }
     }
 
     fn handle_platform_output(&mut self, platform_output: egui::PlatformOutput) {

crates/eframe/src/web/backend.rs

@@ -73,6 +73,10 @@ impl NeedRepaint {
         *repaint_time = repaint_time.min(super::now_sec() + num_seconds);
     }
 
+    pub fn needs_repaint(&self) -> bool {
+        self.when_to_repaint() <= super::now_sec()
+    }
+
     pub fn repaint_asap(&self) {
         *self.0.lock() = f64::NEG_INFINITY;
     }

crates/eframe/src/web/events.rs

@@ -8,22 +8,19 @@ use super::*;
 fn paint_and_schedule(runner_ref: &WebRunner) -> Result<(), JsValue> {
     // Only paint and schedule if there has been no panic
     if let Some(mut runner_lock) = runner_ref.try_lock() {
-        paint_if_needed(&mut runner_lock)?;
+        paint_if_needed(&mut runner_lock);
         drop(runner_lock);
         request_animation_frame(runner_ref.clone())?;
     }
-
     Ok(())
 }
 
-fn paint_if_needed(runner: &mut AppRunner) -> Result<(), JsValue> {
-    if runner.needs_repaint.when_to_repaint() <= now_sec() {
+fn paint_if_needed(runner: &mut AppRunner) {
+    if runner.needs_repaint.needs_repaint() {
         runner.needs_repaint.clear();
-        let clipped_primitives = runner.logic();
-        runner.paint(&clipped_primitives)?;
-        runner.auto_save_if_needed();
+        runner.run_and_paint();
     }
-    Ok(())
+    runner.auto_save_if_needed();
 }
 
 pub(crate) fn request_animation_frame(runner_ref: WebRunner) -> Result<(), JsValue> {
@@ -177,10 +174,14 @@ pub(crate) fn install_document_events(runner_ref: &WebRunner) -> Result<(), JsVa
         "cut",
         |event: web_sys::ClipboardEvent, runner| {
             runner.input.raw.events.push(egui::Event::Cut);
+
             // In Safari we are only allowed to write to the clipboard during the
             // event callback, which is why we run the app logic here and now:
-            runner.logic(); // we ignore the returned triangles, but schedule a repaint right after
+            runner.logic();
+
+            // Make sure we paint the output of the above logic call asap:
             runner.needs_repaint.repaint_asap();
+
             event.stop_propagation();
             event.prevent_default();
         },
@@ -192,10 +193,14 @@ pub(crate) fn install_document_events(runner_ref: &WebRunner) -> Result<(), JsVa
         "copy",
         |event: web_sys::ClipboardEvent, runner| {
             runner.input.raw.events.push(egui::Event::Copy);
+
             // In Safari we are only allowed to write to the clipboard during the
             // event callback, which is why we run the app logic here and now:
-            runner.logic(); // we ignore the returned triangles, but schedule a repaint right after
+            runner.logic();
+
+            // Make sure we paint the output of the above logic call asap:
             runner.needs_repaint.repaint_asap();
+
             event.stop_propagation();
             event.prevent_default();
         },
@@ -281,9 +286,12 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
                     pressed: true,
                     modifiers,
                 });
+
                 // In Safari we are only allowed to write to the clipboard during the
                 // event callback, which is why we run the app logic here and now:
-                runner.logic(); // we ignore the returned triangles, but schedule a repaint right after
+                runner.logic();
+
+                // Make sure we paint the output of the above logic call asap:
                 runner.needs_repaint.repaint_asap();
             }
             event.stop_propagation();
@@ -313,9 +321,12 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
                 pressed: false,
                 modifiers,
             });
+
             // In Safari we are only allowed to write to the clipboard during the
             // event callback, which is why we run the app logic here and now:
-            runner.logic(); // we ignore the returned triangles, but schedule a repaint right after
+            runner.logic();
+
+            // Make sure we paint the output of the above logic call asap:
             runner.needs_repaint.repaint_asap();
 
             text_agent::update_text_agent(runner);

crates/eframe/src/web/mod.rs

@@ -49,6 +49,10 @@ use crate::Theme;
 
 // ----------------------------------------------------------------------------
 
+pub(crate) fn string_from_js_value(value: &JsValue) -> String {
+    value.as_string().unwrap_or_else(|| format!("{value:#?}"))
+}
+
 /// Current time in seconds (since undefined point in time).
 ///
 /// Monotonically increasing.
@@ -196,7 +200,7 @@ fn set_clipboard_text(s: &str) {
             let future = wasm_bindgen_futures::JsFuture::from(promise);
             let future = async move {
                 if let Err(err) = future.await {
-                    log::error!("Copy/cut action failed: {err:?}");
+                    log::error!("Copy/cut action failed: {}", string_from_js_value(&err));
                 }
             };
             wasm_bindgen_futures::spawn_local(future);

crates/eframe/src/web/web_runner.rs

@@ -95,7 +95,10 @@ impl WebRunner {
             log::debug!("Unsubscribing from {} events", events_to_unsubscribe.len());
             for x in events_to_unsubscribe {
                 if let Err(err) = x.unsubscribe() {
-                    log::warn!("Failed to unsubscribe from event: {err:?}");
+                    log::warn!(
+                        "Failed to unsubscribe from event: {}",
+                        super::string_from_js_value(&err)
+                    );
                 }
             }
         }