Fix blurry rendering in some browsers (#4299)

* Closes https://github.com/emilk/egui/issues/4241

I would love some more testers of this.

I'm not sure if we really need the round-to-even code, but I'm hesitant
to out-right revert https://github.com/emilk/egui/pull/151 when I cannot
reproduce its problem. Keeping it seems quite safe though.

---
# Testing
Checkout the branch and run:

* `./scripts/start_server.sh`
* `./scripts/build_demo_web.sh` and then open
`http://localhost:8888/index.html#Rendering`
* `./scripts/build_demo_web.sh --wgpu` and then open
`http://localhost:8888/index.html#Rendering`

Check the "Rendering test" that the squares in the pixel alignment test
are perfectly sharp, like this:

<img width="576" alt="Screenshot 2024-04-01 at 13 27 20"
src="https://github.com/emilk/egui/assets/1148717/fb6c4824-9e25-4304-bc0c-3c50fbd44a52">

If it looks something like this, something is WRONG:
<img width="488" alt="Screenshot 2024-04-01 at 13 29 07"
src="https://github.com/emilk/egui/assets/1148717/04bd93ff-2108-40c5-95f6-76e3bcb9cd7f">


Please try it on different zoom levels in different browsers, and if
possible on different monitors with different native dpi scaling. Report
back the results!


### Mac
I have tested on a high-DPI Mac:
* Chromium (Brave): ✅ Can reproduce problem on `master`, and it's now
fixed
* Firefox:  ✅ Can reproduce problem on `master`, and it's now fixed
* Safari: ❌ Can't get it to work; giving up for now

crates/eframe/src/web/mod.rs

@@ -130,51 +130,46 @@ fn resize_canvas_to_screen_size(
 ) -> 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 width = parent.client_width();
-    let height = parent.client_height();
-
-    let canvas_real_size = Vec2 {
-        x: width as f32,
-        y: height as f32,
+    let parent_size_points = Vec2 {
+        x: parent.client_width() as f32,
+        y: parent.client_height() as f32,
     };
 
-    if width <= 0 || height <= 0 {
-        log::error!("egui canvas parent size is {}x{}. Try adding `html, body {{ height: 100%; width: 100% }}` to your CSS!", width, height);
+    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);
     }
 
-    let pixels_per_point = native_pixels_per_point();
-
-    let max_size_pixels = pixels_per_point * max_size_points;
+    // 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 * canvas_real_size;
-    let canvas_size_pixels = canvas_size_pixels.min(max_size_pixels);
-    let canvas_size_points = canvas_size_pixels / pixels_per_point;
+    let canvas_size_pixels = pixels_per_point * parent_size_points.min(max_size_points);
 
-    // Make sure that the height and width are always even numbers.
+    // 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
-    fn round_to_even(v: f32) -> f32 {
-        (v / 2.0).round() * 2.0
-    }
+    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", round_to_even(canvas_size_points.x)),
-        )
+        .set_property("width", &format!("{}px", canvas_size_points.x))
         .ok()?;
     canvas
         .style()
-        .set_property(
-            "height",
-            &format!("{}px", round_to_even(canvas_size_points.y)),
-        )
+        .set_property("height", &format!("{}px", canvas_size_points.y))
         .ok()?;
-    canvas.set_width(round_to_even(canvas_size_pixels.x) as u32);
-    canvas.set_height(round_to_even(canvas_size_pixels.y) as u32);
+    canvas.set_width(canvas_size_pixels.x as u32);
+    canvas.set_height(canvas_size_pixels.y as u32);
 
     Some(())
 }

web_demo/index.html

@@ -37,7 +37,12 @@
             width: 100%;
         }
 
-        /* Position canvas in center-top: */
+        /* Position canvas in center-top.
+        This is rather arbitrarily chosen.
+        In particular, it seems like both Chromium and Firefox will still align
+        the canvas on the physical pixel grid, which is required to get
+        pixel-perfect (non-blurry) rendering in egui.
+        See https://github.com/emilk/egui/issues/4241 for more */
         canvas {
             margin-right: auto;
             margin-left: auto;