Add egui_kittest::HarnessBuilder::with_os and set the default to Nix (#7493)

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_kittest/src/builder.rs

@@ -8,6 +8,7 @@ pub struct HarnessBuilder<State = ()> {
     pub(crate) screen_rect: Rect,
     pub(crate) pixels_per_point: f32,
     pub(crate) theme: egui::Theme,
+    pub(crate) os: egui::os::OperatingSystem,
     pub(crate) max_steps: u64,
     pub(crate) step_dt: f32,
     pub(crate) state: PhantomData<State>,
@@ -26,6 +27,7 @@ impl<State> Default for HarnessBuilder<State> {
             max_steps: 4,
             step_dt: 1.0 / 4.0,
             wait_for_pending_images: true,
+            os: egui::os::OperatingSystem::Nix,
         }
     }
 }
@@ -54,6 +56,21 @@ impl<State> HarnessBuilder<State> {
         self
     }
 
+    /// Override the [`egui::os::OperatingSystem`] reported to egui.
+    ///
+    /// This affects e.g. the way shortcuts are displayed. So for snapshot tests,
+    /// it makes sense to set this to a specific OS, so snapshots don't change when running
+    /// the same tests on different OSes.
+    ///
+    /// Default is [`egui::os::OperatingSystem::Nix`].
+    /// Use [`egui::os::OperatingSystem::from_target_os()`] to use the current OS (this restores
+    /// eguis default behavior).
+    #[inline]
+    pub fn with_os(mut self, os: egui::os::OperatingSystem) -> Self {
+        self.os = os;
+        self
+    }
+
     /// Set the maximum number of steps to run when calling [`Harness::run`].
     ///
     /// Default is 4.

crates/egui_kittest/src/lib.rs

@@ -93,6 +93,7 @@ impl<'a, State> Harness<'a, State> {
             screen_rect,
             pixels_per_point,
             theme,
+            os,
             max_steps,
             step_dt,
             state: _,
@@ -101,6 +102,7 @@ impl<'a, State> Harness<'a, State> {
         } = builder;
         let ctx = ctx.unwrap_or_default();
         ctx.set_theme(theme);
+        ctx.set_os(os);
         ctx.enable_accesskit();
         ctx.all_styles_mut(|style| {
             // Disable cursor blinking so it doesn't interfere with snapshots