diff --git a/Cargo.lock b/Cargo.lock
index 43a25113bd8..eb32a0c1a1a 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1388,6 +1388,7 @@ dependencies = [
  "document-features",
  "egui",
  "egui-wgpu",
+ "egui_kittest",
  "image",
  "kittest",
  "pollster",
diff --git a/crates/egui_demo_lib/src/demo/widget_gallery.rs b/crates/egui_demo_lib/src/demo/widget_gallery.rs
index 60c9bccf0a9..5dfe27d3c1b 100644
--- a/crates/egui_demo_lib/src/demo/widget_gallery.rs
+++ b/crates/egui_demo_lib/src/demo/widget_gallery.rs
@@ -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()
-            .with_size(Vec2::new(380.0, 550.0))
+        let mut harness = Harness::builder()
             .with_dpi(2.0)
-            .build(app);
+            .with_size(Vec2::new(380.0, 550.0))
+            .build_ui(|ui| demo.ui(ui));
+
+        harness.fit_contents();
 
         harness.wgpu_snapshot("widget_gallery");
     }
diff --git a/crates/egui_demo_lib/src/rendering_test.rs b/crates/egui_demo_lib/src/rendering_test.rs
index 3ae4f111808..a5a8dde7c99 100644
--- a/crates/egui_demo_lib/src/rendering_test.rs
+++ b/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 +677,36 @@ 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_dpi(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_{:.2}", dpi));
+            if let Err(err) = result {
+                errors.push(err);
+            }
+        }
+
+        if !errors.is_empty() {
+            panic!("Errors: {:#?}", errors);
+        }
+    }
+}
diff --git a/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.00.png b/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.00.png
new file mode 100644
index 00000000000..e5870bd8fe3
--- /dev/null
+++ b/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:bf774de50f485096b1190093e2ef9a9e598f1d19941045386f182b7e51744d0c
+size 277095
diff --git a/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.25.png b/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.25.png
new file mode 100644
index 00000000000..fc42214700b
--- /dev/null
+++ b/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 sha256:1cf07a6f55057730133d192bcbf26d95583377d8c6147f3788eefa2d527972d4
+size 375597
diff --git a/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.50.png b/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.50.png
new file mode 100644
index 00000000000..4d5ec243d56
--- /dev/null
+++ b/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 sha256:06bc9f9adfbd017ae51cf1be8910db9db4552d6e741205065832cc29654bd38d
+size 462813
diff --git a/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.67.png b/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.67.png
new file mode 100644
index 00000000000..3f1d5c8f417
--- /dev/null
+++ b/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 sha256:567d52eb9b96942670e920c6e88bbfb5adc2690f6d5a2b6b87996ff96584899e
+size 533920
diff --git a/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.75.png b/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_1.75.png
new file mode 100644
index 00000000000..5db5db3a7fa
--- /dev/null
+++ b/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 sha256:5837945b9b8f60a6dbc334eb84eb0df6217f240ca7fda3a12f6c79ba21a23ac1
+size 567531
diff --git a/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_2.00.png b/crates/egui_demo_lib/tests/snapshots/rendering_test/dpi_2.00.png
new file mode 100644
index 00000000000..26eb302bfd8
--- /dev/null
+++ b/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 sha256:06de15c3e72805174fd9efc3ec9697b6360d3f54d67f2e837630c6887aeb1c15
+size 659759
diff --git a/crates/egui_demo_lib/tests/snapshots/widget_gallery.png b/crates/egui_demo_lib/tests/snapshots/widget_gallery.png
index 273b85a6303..8aa817094cd 100644
--- a/crates/egui_demo_lib/tests/snapshots/widget_gallery.png
+++ b/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 sha256:3833dc1709543abbb320ce32f47880bd40299801fa633c7f0eee27bed643e5ab
+size 158260
diff --git a/crates/egui_kittest/Cargo.toml b/crates/egui_kittest/Cargo.toml
index 5063b59658b..f9f98198c82 100644
--- a/crates/egui_kittest/Cargo.toml
+++ b/crates/egui_kittest/Cargo.toml
@@ -39,6 +39,7 @@ dify = { workspace = true, optional = true }
 document-features = { workspace = true, optional = true }
 
 [dev-dependencies]
+egui_kittest = { workspace = true, features = ["wgpu", "snapshot"] }
 wgpu = { workspace = true, features = ["metal"] }
 image = { workspace = true, features = ["png"] }
 egui = { workspace = true, features = ["default_fonts"] }
diff --git a/crates/egui_kittest/src/builder.rs b/crates/egui_kittest/src/builder.rs
index bf1daf3622b..381cea1e625 100644
--- a/crates/egui_kittest/src/builder.rs
+++ b/crates/egui_kittest/src/builder.rs
@@ -1,3 +1,4 @@
+use crate::harness_kind::AppKind;
 use crate::Harness;
 use egui::{Pos2, Rect, Vec2};
 
@@ -50,6 +51,10 @@ impl HarnessBuilder {
     ///     });
     /// ```
     pub fn build<'a>(self, app: impl FnMut(&egui::Context) + 'a) -> Harness<'a> {
-        Harness::from_builder(&self, app)
+        Harness::from_builder(&self, AppKind::Context(Box::new(app)))
+    }
+
+    pub fn build_ui<'a>(self, app: impl FnMut(&mut egui::Ui) + 'a) -> Harness<'a> {
+        Harness::from_builder(&self, AppKind::Ui(Box::new(app)))
     }
 }
diff --git a/crates/egui_kittest/src/harness_kind.rs b/crates/egui_kittest/src/harness_kind.rs
new file mode 100644
index 00000000000..25e37f6b6c4
--- /dev/null
+++ b/crates/egui_kittest/src/harness_kind.rs
@@ -0,0 +1,72 @@
+use egui::Frame;
+
+type AppKindContext<'a> = Box<dyn FnMut(&egui::Context) + 'a>;
+type AppKindUi<'a> = Box<dyn FnMut(&mut egui::Ui) + 'a>;
+
+pub enum AppKind<'a> {
+    Context(AppKindContext<'a>),
+    Ui(AppKindUi<'a>),
+}
+
+// TODO: These aren't working unfortunately :(
+// I think they should work though: https://geo-ant.github.io/blog/2021/rust-traits-and-variadic-functions/
+// pub trait IntoAppKind<'a, UiKind> {
+//     fn into_harness_kind(self) -> AppKind<'a>;
+// }
+//
+// impl<'a, F> IntoAppKind<'a, &egui::Context> for F
+// where
+//     F: FnMut(&egui::Context) + 'a,
+// {
+//     fn into_harness_kind(self) -> AppKind<'a> {
+//         AppKind::Context(Box::new(self))
+//     }
+// }
+//
+// impl<'a, F> IntoAppKind<'a, &mut egui::Ui> for F
+// where
+//     F: FnMut(&mut egui::Ui) + 'a,
+// {
+//     fn into_harness_kind(self) -> AppKind<'a> {
+//         AppKind::Ui(Box::new(self))
+//     }
+// }
+
+impl<'a> AppKind<'a> {
+    pub fn run(&mut self, ctx: &egui::Context) -> Option<egui::Response> {
+        match self {
+            AppKind::Context(f) => {
+                f(ctx);
+                None
+            }
+            AppKind::Ui(f) => Some(Self::run_ui(f, ctx, false)),
+        }
+    }
+
+    pub fn run_sizing_pass(&mut self, ctx: &egui::Context) -> Option<egui::Response> {
+        match self {
+            AppKind::Context(f) => {
+                f(ctx);
+                None
+            }
+            AppKind::Ui(f) => Some(Self::run_ui(f, ctx, true)),
+        }
+    }
+
+    fn run_ui(f: &mut AppKindUi<'a>, ctx: &egui::Context, sizing_pass: bool) -> egui::Response {
+        egui::CentralPanel::default()
+            .frame(
+                Frame::central_panel(&ctx.style())
+                    .inner_margin(0.0)
+                    .outer_margin(0.0),
+            )
+            .show(ctx, |ui| {
+                let mut builder = egui::UiBuilder::new();
+                if sizing_pass {
+                    builder.sizing_pass = true;
+                }
+                ui.scope_builder(builder, f).response
+            })
+            .inner
+    }
+}
diff --git a/crates/egui_kittest/src/lib.rs b/crates/egui_kittest/src/lib.rs
index f4f00ad652f..7ad9b4aa880 100644
--- a/crates/egui_kittest/src/lib.rs
+++ b/crates/egui_kittest/src/lib.rs
@@ -11,6 +11,7 @@ mod snapshot;
 #[cfg(feature = "snapshot")]
 pub use snapshot::*;
 use std::fmt::{Debug, Formatter};
+pub mod harness_kind;
 #[cfg(feature = "wgpu")]
 mod texture_to_image;
 #[cfg(feature = "wgpu")]
@@ -20,8 +21,9 @@ pub use kittest;
 use std::mem;
 
 use crate::event::EventState;
+use crate::harness_kind::AppKind;
 pub use builder::*;
-use egui::{Pos2, Rect, TexturesDelta, Vec2, ViewportId};
+use egui::{CentralPanel, Pos2, Rect, TexturesDelta, UiBuilder, Vec2, ViewportId};
 use kittest::{Node, Queryable};
 
 /// The test Harness. This contains everything needed to run the test.
@@ -32,8 +34,9 @@ pub struct Harness<'a> {
     kittest: kittest::State,
     output: egui::FullOutput,
     texture_deltas: Vec<TexturesDelta>,
-    update_fn: Box<dyn FnMut(&egui::Context) + 'a>,
+    app: AppKind<'a>,
     event_state: EventState,
+    response: Option<egui::Response>,
 }
 
 impl<'a> Debug for Harness<'a> {
@@ -43,10 +46,7 @@ impl<'a> Debug for Harness<'a> {
 }
 
 impl<'a> Harness<'a> {
-    pub(crate) fn from_builder(
-        builder: &HarnessBuilder,
-        mut app: impl FnMut(&egui::Context) + 'a,
-    ) -> Self {
+    pub(crate) fn from_builder(builder: &HarnessBuilder, mut app: AppKind<'a>) -> Self {
         let ctx = egui::Context::default();
         ctx.enable_accesskit();
         let mut input = egui::RawInput {
@@ -56,12 +56,16 @@ impl<'a> Harness<'a> {
         let viewport = input.viewports.get_mut(&ViewportId::ROOT).unwrap();
         viewport.native_pixels_per_point = Some(builder.dpi);
 
+        let mut response = None;
+
         // We need to run egui for a single frame so that the AccessKit state can be initialized
         // and users can immediately start querying for widgets.
-        let mut output = ctx.run(input.clone(), &mut app);
+        let mut output = ctx.run(input.clone(), |ctx| {
+            response = app.run(ctx);
+        });
 
         let mut harness = Self {
-            update_fn: Box::new(app),
+            app,
             ctx,
             input,
             kittest: kittest::State::new(
@@ -73,6 +77,7 @@ impl<'a> Harness<'a> {
             ),
             texture_deltas: vec![mem::take(&mut output.textures_delta)],
             output,
+            response,
             event_state: EventState::default(),
         };
         // Run the harness until it is stable, ensuring that all Areas are shown and animations are done
@@ -104,6 +109,10 @@ impl<'a> Harness<'a> {
         Self::builder().build(app)
     }
 
+    pub fn new_ui(app: impl FnMut(&mut egui::Ui) + 'a) -> Self {
+        Self::builder().build_ui(app)
+    }
+
     /// Set the size of the window.
     /// Note: If you only want to set the size once at the beginning,
     /// prefer using [`HarnessBuilder::with_size`].
@@ -125,13 +134,23 @@ impl<'a> Harness<'a> {
     /// Run a frame.
     /// This will call the app closure with the current context and update the Harness.
     pub fn step(&mut self) {
+        self._step(false);
+    }
+
+    fn _step(&mut self, sizing_pass: bool) {
         for event in self.kittest.take_events() {
             if let Some(event) = self.event_state.kittest_event_to_egui(event) {
                 self.input.events.push(event);
             }
         }
 
-        let mut output = self.ctx.run(self.input.take(), self.update_fn.as_mut());
+        let mut output = self.ctx.run(self.input.take(), |ctx| {
+            if sizing_pass {
+                self.response = self.app.run_sizing_pass(ctx);
+            } else {
+                self.response = self.app.run(ctx);
+            }
+        });
         self.kittest.update(
             output
                 .platform_output
@@ -144,6 +163,14 @@ impl<'a> Harness<'a> {
         self.output = output;
     }
 
+    pub fn fit_contents(&mut self) {
+        self._step(true);
+        if let Some(response) = &self.response {
+            self.set_size(response.rect.size());
+        }
+        self.run();
+    }
+
     /// Run a few frames.
     /// This will soon be changed to run the app until it is "stable", meaning
     /// - all animations are done
diff --git a/crates/egui_kittest/tests/snapshots/test_shrink.png b/crates/egui_kittest/tests/snapshots/test_shrink.png
new file mode 100644
index 00000000000..25dbe1172b1
--- /dev/null
+++ b/crates/egui_kittest/tests/snapshots/test_shrink.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:d03cf541a9e281bae54de648ddb43faffb054e6472ce56387ca720dd81ef905a
+size 13640
diff --git a/crates/egui_kittest/tests/tests.rs b/crates/egui_kittest/tests/tests.rs
new file mode 100644
index 00000000000..4978cbeb775
--- /dev/null
+++ b/crates/egui_kittest/tests/tests.rs
@@ -0,0 +1,14 @@
+use egui_kittest::Harness;
+
+#[test]
+fn test_shrink() {
+    let mut harness = Harness::new_ui(|ui| {
+        ui.label("Hello, world!");
+        ui.separator();
+        ui.label("This is a test");
+    });
+
+    harness.fit_contents();
+
+    harness.wgpu_snapshot("test_shrink");
+}