swrenderer: intersect the dirty region with the screen rectangle

When rounded out, the dirty region might end up bigger than the size
of the screen, which would then result in panic while trying to draw a
pixel out of the screen.

Fixes #6932

api/rs/slint/tests/partial_renderer.rs

@@ -262,3 +262,22 @@ fn list_view() {
     }));
     assert!(!window.draw_if_needed(|_| { unreachable!() }));
 }
+
+#[test]
+/// test for #6932
+fn scale_factor() {
+    slint::slint! {
+        export component Ui inherits Window {
+        }
+    }
+
+    slint::platform::set_platform(Box::new(TestPlatform)).ok();
+    let ui = Ui::new().unwrap();
+    let window = WINDOW.with(|x| x.clone());
+    window.set_size(slint::PhysicalSize::new(500, 500));
+    window.dispatch_event(slint::platform::WindowEvent::ScaleFactorChanged { scale_factor: 1.33 });
+    ui.show().unwrap();
+    assert!(window.draw_if_needed(|renderer| {
+        do_test_render_region(renderer, 0, 0, 500, 500);
+    }));
+}

internal/core/software_renderer.rs

@@ -503,8 +503,15 @@ impl SoftwareRenderer {
                     logical_size,
                 );
                 let rotation = RotationInfo { orientation: rotation, screen_size: size };
-                let mut i = renderer.dirty_region.iter().map(|r| {
-                    (r.cast() * factor).to_rect().round_out().cast().transformed(rotation)
+                let screen_rect = PhysicalRect::from_size(size);
+                let mut i = renderer.dirty_region.iter().filter_map(|r| {
+                    (r.cast() * factor)
+                        .to_rect()
+                        .round_out()
+                        .cast()
+                        .intersection(&screen_rect)?
+                        .transformed(rotation)
+                        .into()
                 });
                 let dirty_region = PhysicalRegion {
                     rectangles: core::array::from_fn(|_| i.next().unwrap_or_default().to_box2d()),
@@ -973,10 +980,16 @@ fn prepare_scene(
         );
         let rotation =
             RotationInfo { orientation: software_renderer.rotation.get(), screen_size: size };
-        let mut i = renderer
-            .dirty_region
-            .iter()
-            .map(|r| (r.cast() * factor).to_rect().round_out().cast().transformed(rotation));
+        let screen_rect = PhysicalRect::from_size(size);
+        let mut i = renderer.dirty_region.iter().filter_map(|r| {
+            (r.cast() * factor)
+                .to_rect()
+                .round_out()
+                .cast()
+                .intersection(&screen_rect)?
+                .transformed(rotation)
+                .into()
+        });
         dirty_region = PhysicalRegion {
             rectangles: core::array::from_fn(|_| i.next().unwrap_or_default().to_box2d()),
             count: renderer.dirty_region.iter().count(),