Add Harness::new_ui, Harness::fit_contents (#5301)

This adds a `Harness::new_ui`, which accepts a Ui closure and shows the
ui in a central panel. One big benefit is that this allows us to add a
fit_contents method that can run the ui closure with a sizing pass and
resize the "screen" based on the content size.

I also used this to add a snapshot test for the rendering_test at
different scales.

crates/egui_demo_lib/src/demo/widget_gallery.rs

@@ -290,7 +290,7 @@ fn doc_link_label_with_crate<'a>(
 mod tests {
     use super::*;
     use crate::View;
-    use egui::{CentralPanel, Context, Vec2};
+    use egui::Vec2;
     use egui_kittest::Harness;
 
     #[test]
@@ -300,15 +300,12 @@ mod tests {
             date: Some(chrono::NaiveDate::from_ymd_opt(2024, 1, 1).unwrap()),
             ..Default::default()
         };
-        let app = |ctx: &Context| {
-            CentralPanel::default().show(ctx, |ui| {
-                demo.ui(ui);
-            });
-        };
-        let harness = Harness::builder()
+        let mut harness = Harness::builder()
+            .with_pixels_per_point(2.0)
             .with_size(Vec2::new(380.0, 550.0))
-            .with_dpi(2.0)
-            .build(app);
+            .build_ui(|ui| demo.ui(ui));
+
+        harness.fit_contents();
 
         harness.wgpu_snapshot("widget_gallery");
     }

crates/egui_demo_lib/src/rendering_test.rs

@@ -435,10 +435,7 @@ fn pixel_test_strokes(ui: &mut Ui) {
         let thickness_pixels = thickness_pixels as f32;
         let thickness_points = thickness_pixels / pixels_per_point;
         let num_squares = (pixels_per_point * 10.0).round().max(10.0) as u32;
-        let size_pixels = vec2(
-            ui.available_width(),
-            num_squares as f32 + thickness_pixels * 2.0,
-        );
+        let size_pixels = vec2(ui.min_size().x, num_squares as f32 + thickness_pixels * 2.0);
         let size_points = size_pixels / pixels_per_point + Vec2::splat(2.0);
         let (response, painter) = ui.allocate_painter(size_points, Sense::hover());
 
@@ -680,3 +677,34 @@ fn mul_color_gamma(left: Color32, right: Color32) -> Color32 {
         (left.a() as f32 * right.a() as f32 / 255.0).round() as u8,
     )
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::ColorTest;
+    use egui::vec2;
+
+    #[test]
+    pub fn rendering_test() {
+        let mut errors = vec![];
+        for dpi in [1.0, 1.25, 1.5, 1.75, 1.6666667, 2.0] {
+            let mut color_test = ColorTest::default();
+            let mut harness = egui_kittest::Harness::builder()
+                .with_size(vec2(2000.0, 2000.0))
+                .with_pixels_per_point(dpi)
+                .build_ui(|ui| {
+                    color_test.ui(ui);
+                });
+
+            //harness.set_size(harness.ctx.used_size());
+
+            harness.fit_contents();
+
+            let result = harness.try_wgpu_snapshot(&format!("rendering_test/dpi_{dpi:.2}"));
+            if let Err(err) = result {
+                errors.push(err);
+            }
+        }
+
+        assert!(errors.is_empty(), "Errors: {errors:#?}");
+    }
+}

crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.00.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:023eaa363b42ec24ae845dc2ca9ff271a0bd47217e625785d3716044ecfa7a64
+size 278444

crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.25.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:d81f618e54176b1c43b710121f249e13ce29827fbea3451827ab62229006677e
+size 378603

crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.50.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:2d8eca6d5555ef779233175615b877fb91318b4a09a37e5cfbe71973d56f4caf
+size 465907

crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.67.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:4768804f57dfc54c5f6b84a2686038b8d630a28c7e928ae044d5b2ce8377e2cd
+size 538775

crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.75.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:fcee0e0302f33d681348d62bee3b548beb494c6dd1fa3454586986e0b699e162
+size 572403

crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_2.00.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:254a8dff0b1d4b74971fd3bd4044c4ec0ce49412a95e98419a14dc55b32a4fc9
+size 663272

crates/egui_demo_lib/tests/snapshots/widget_gallery.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:5dc632962f8894c4f20a48c9b9e57d60470f3f83ef7f19d05854dba718610a2f
-size 161820
+oid sha256:c069ef4f86beeeafd8686f30fc914bedd7e7e7ec38fd96e9a46ac6b31308c43f
+size 160883