Make sure egui can handle zero-sized screen rect

This would previously hit a debug assert

Fixes https://github.com/emilk/egui/issues/395

egui/src/containers/frame.rs

@@ -121,7 +121,12 @@ impl Frame {
     pub fn begin(self, ui: &mut Ui) -> Prepared {
         let where_to_put_background = ui.painter().add(Shape::Noop);
         let outer_rect_bounds = ui.available_rect_before_wrap();
-        let inner_rect = outer_rect_bounds.shrink2(self.margin);
+        let mut inner_rect = outer_rect_bounds.shrink2(self.margin);
+
+        // Make sure we don't shrink to the negative:
+        inner_rect.max.x = inner_rect.max.x.max(inner_rect.min.x);
+        inner_rect.max.y = inner_rect.max.y.max(inner_rect.min.y);
+
         let content_ui = ui.child_ui(inner_rect, *ui.layout());
 
         // content_ui.set_clip_rect(outer_rect_bounds.shrink(self.stroke.width * 0.5)); // Can't do this since we don't know final size yet

egui_demo_lib/src/lib.rs

@@ -139,3 +139,22 @@ fn test_egui_e2e() {
         assert!(!clipped_meshes.is_empty());
     }
 }
+
+#[test]
+fn test_egui_zero_window_size() {
+    let mut demo_windows = crate::DemoWindows::default();
+    let mut ctx = egui::CtxRef::default();
+    let raw_input = egui::RawInput {
+        screen_rect: Some(egui::Rect::from_min_max(egui::Pos2::ZERO, egui::Pos2::ZERO)),
+        ..Default::default()
+    };
+
+    const NUM_FRAMES: usize = 5;
+    for _ in 0..NUM_FRAMES {
+        ctx.begin_frame(raw_input.clone());
+        demo_windows.ui(&ctx);
+        let (_output, shapes) = ctx.end_frame();
+        let clipped_meshes = ctx.tessellate(shapes);
+        assert!(clipped_meshes.is_empty(), "There should be nothing to show");
+    }
+}