From d06f5b5dfcd714811ea0ae590cb410771d0706c1 Mon Sep 17 00:00:00 2001
From: Lucas Meurer <hi@lucasmerlin.me>
Date: Tue, 28 Jul 2026 10:54:45 +0200
Subject: [PATCH] Handle `ViewportCommand::InnerSize` in `egui_kittest` (#8350)

Useful to support resizing headless apps via `egui_inspection`
---
 crates/egui_kittest/src/lib.rs     | 40 +++++++++++++++++++++++++-----
 crates/egui_kittest/tests/tests.rs | 31 +++++++++++++++++++++++
 2 files changed, 65 insertions(+), 6 deletions(-)

diff --git a/crates/egui_kittest/src/lib.rs b/crates/egui_kittest/src/lib.rs
index 4f691c8c4..6873be579 100644
--- a/crates/egui_kittest/src/lib.rs
+++ b/crates/egui_kittest/src/lib.rs
@@ -175,10 +175,9 @@ impl<'a, State> Harness<'a, State> {
             #[cfg(feature = "snapshot")]
             snapshot_results: SnapshotResults::default(),
         };
-        // Fulfill any screenshot requested during the initial frame above (which didn't go
-        // through `_step`).
-        #[cfg(any(feature = "wgpu", feature = "snapshot"))]
-        harness.handle_screenshots();
+        // Handle any viewport commands (e.g. a screenshot or resize) requested during the initial
+        // frame above (which didn't go through `_step`).
+        harness.handle_viewport_commands();
 
         // Run the harness until it is stable, ensuring that all Areas are shown and animations are done
         harness.run_ok();
@@ -273,8 +272,7 @@ impl<'a, State> Harness<'a, State> {
         self.renderer.handle_delta(&output.textures_delta);
         self.output = output;
 
-        #[cfg(any(feature = "wgpu", feature = "snapshot"))]
-        self.handle_screenshots();
+        self.handle_viewport_commands();
     }
 
     /// Calculate the rect that includes all popups and tooltips.
@@ -668,6 +666,36 @@ impl<'a, State> Harness<'a, State> {
         self.renderer.render(&self.ctx, &output)
     }
 
+    /// Apply the [`egui::ViewportCommand`]s the app emitted during the last frame.
+    fn handle_viewport_commands(&mut self) {
+        self.handle_inner_size();
+
+        #[cfg(any(feature = "wgpu", feature = "snapshot"))]
+        self.handle_screenshots();
+    }
+
+    /// Resize the harness to the last [`egui::ViewportCommand::InnerSize`] requested by the app
+    /// during the last frame, if any.
+    fn handle_inner_size(&mut self) {
+        let new_inner_size =
+            self.root_viewport_output()
+                .commands
+                .iter()
+                .rev()
+                .find_map(|command| {
+                    if let egui::ViewportCommand::InnerSize(size) = command {
+                        Some(*size)
+                    } else {
+                        None
+                    }
+                });
+
+        if let Some(size) = new_inner_size {
+            self.set_size(size);
+            self.ctx.request_repaint();
+        }
+    }
+
     /// Fulfill any [`egui::ViewportCommand::Screenshot`] requests made by the app during the
     /// last frame.
     ///
diff --git a/crates/egui_kittest/tests/tests.rs b/crates/egui_kittest/tests/tests.rs
index 37d88da9a..9bdd2b6c1 100644
--- a/crates/egui_kittest/tests/tests.rs
+++ b/crates/egui_kittest/tests/tests.rs
@@ -262,3 +262,34 @@ fn test_ime_composition_visuals() {
     harness.run();
     harness.snapshot("test_ime_composition_visuals_cursor");
 }
+
+#[test]
+fn inner_size_viewport_command() {
+    let new_size = Vec2::new(300.0, 200.0);
+
+    #[derive(Default)]
+    struct State {
+        requested: bool,
+        observed_size: Option<Vec2>,
+    }
+
+    let mut harness = Harness::builder()
+        .with_size(Vec2::new(100.0, 80.0))
+        .build_ui_state(
+            |ui, state: &mut State| {
+                // Request the resize once.
+                if !state.requested {
+                    state.requested = true;
+                    ui.ctx()
+                        .send_viewport_cmd(egui::ViewportCommand::InnerSize(new_size));
+                }
+
+                state.observed_size = Some(ui.ctx().viewport_rect().size());
+            },
+            State::default(),
+        );
+
+    harness.run();
+
+    assert_eq!(harness.state().observed_size, Some(new_size));
+}