Add test

crates/egui_kittest/tests/atom_layout.rs

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+#![cfg(feature = "snapshot")]
+#![cfg(feature = "wgpu")]
+
+use egui::widget_style::{Classes, WidgetState};
+use egui::{Atom, AtomExt, AtomLayout, Direction, Frame};
+
+/// A root [`AtomLayout`] (in a [`Frame::canvas`]) that stacks several nested [`AtomLayout`]s
+/// (each in an inactive button frame), one per [`Direction`], to exercise nesting and the
+/// direction setting together.
+#[test]
+fn atom_layout_nesting_and_direction() {
+    let mut harness = egui_kittest::Harness::new_ui(|ui| {
+        let style = ui.style().clone();
+
+        // The frame of an inactive button.
+        let button_frame = style
+            .button_style(&Classes::default(), WidgetState::Inactive)
+            .frame;
+
+        // A nested layout laid out along `direction`, labelled to read in that direction.
+        let row = |direction: Direction, atoms: [&'static str; 3]| {
+            Atom::layout(
+                AtomLayout::new((atoms[0], atoms[1], atoms[2]))
+                    .direction(direction)
+                    .frame(button_frame.clone()),
+            )
+        };
+
+        // Each axis pair gets the same label order, so the reversed direction visibly flips it
+        // (e.g. `RightToLeft` reads "right to left").
+        AtomLayout::new((
+            // The two horizontal rows stacked into their own `TopDown` layout.
+            Atom::layout(
+                AtomLayout::new((
+                    row(Direction::LeftToRight, ["left", "to", "right"]).atom_grow(true),
+                    row(Direction::RightToLeft, ["left", "to", "right"]).atom_grow(true),
+                ))
+                .direction(Direction::TopDown),
+            ).atom_grow(true),
+            row(Direction::TopDown, ["top", "to", "bottom"]),
+            row(Direction::BottomUp, ["top", "to", "bottom"]),
+        ))
+        .direction(Direction::LeftToRight)
+        .frame(Frame::canvas(&style))
+        .show(ui);
+    });
+
+    harness.fit_contents();
+
+    #[cfg(all(feature = "snapshot", feature = "wgpu"))]
+    harness.snapshot("atom_layout_nesting");
+}

crates/egui_kittest/tests/snapshots/atom_layout_nesting.png

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+size 6338