fit layer buffers during output resize

compositor/src/winit.rs

@@ -1452,26 +1452,30 @@ fn layer_elements(
 
     let mut elements = Vec::new();
     for (geometry, layer) in layers {
-        elements.extend(
-            AsRenderElements::<GlesRenderer>::render_elements::<
-                WaylandSurfaceRenderElement<GlesRenderer>,
-            >(
-                &layer,
-                renderer,
-                geometry.loc.to_physical_precise_round(scale),
-                scale.into(),
-                1.0,
-            )
-            .into_iter()
-            .filter_map(|element| {
-                let origin = Point::<i32, Physical>::default();
-                let element = RescaleRenderElement::from_element(element, origin, 1.0);
-                let element =
-                    RelocateRenderElement::from_element(element, origin, Relocate::Relative);
+        let target = geometry.to_physical_precise_round(scale);
+        let surface_elements = AsRenderElements::<GlesRenderer>::render_elements::<
+            WaylandSurfaceRenderElement<GlesRenderer>,
+        >(&layer, renderer, target.loc, scale.into(), 1.0);
+        let source_bounds = surface_elements
+            .iter()
+            .map(|element| element.geometry(scale.into()))
+            .reduce(Rectangle::merge);
+        let content_scale = source_bounds
+            .map(|source| layer_content_scale(source.size, target.size))
+            .unwrap_or_else(|| RenderScale::from(1.0));
+        let scale_origin = source_bounds.map_or(target.loc, |source| source.loc);
+        let crop = target.intersection(output_crop).unwrap_or_default();
 
-                CropRenderElement::from_element(element, scale, output_crop).map(Into::into)
-            }),
-        );
+        elements.extend(surface_elements.into_iter().filter_map(|element| {
+            let element = RescaleRenderElement::from_element(element, scale_origin, content_scale);
+            let element = RelocateRenderElement::from_element(
+                element,
+                Point::<i32, Physical>::default(),
+                Relocate::Relative,
+            );
+
+            CropRenderElement::from_element(element, scale, crop).map(Into::into)
+        }));
 
         if let Some(material) = material_element(state, renderer, output, &layer, geometry) {
             elements.push(material);
@@ -1481,6 +1485,20 @@ fn layer_elements(
     elements
 }
 
+fn layer_content_scale(
+    source: Size<i32, Physical>,
+    target: Size<i32, Physical>,
+) -> RenderScale<f64> {
+    if source.w <= 0 || source.h <= 0 || target.w <= 0 || target.h <= 0 {
+        return RenderScale::from(1.0);
+    }
+
+    RenderScale::from((
+        f64::from(target.w) / f64::from(source.w),
+        f64::from(target.h) / f64::from(source.h),
+    ))
+}
+
 fn material_element(
     state: &mut Ferese,
     renderer: &mut GlesRenderer,
@@ -1813,7 +1831,8 @@ mod tests {
 
     use super::{
         color_with_alpha, expanded_blur_region, framebuffer_capture_rect, framebuffer_clip_rect,
-        normalized_scale, rounded_visual_rect, scaled_visual_rect, shadow_bounds,
+        layer_content_scale, normalized_scale, rounded_visual_rect, scaled_visual_rect,
+        shadow_bounds,
     };
 
     #[derive(Debug)]
@@ -1866,6 +1885,21 @@ mod tests {
         assert_eq!(normalized_scale(f64::INFINITY), 1.0);
     }
 
+    #[test]
+    fn layer_content_tracks_new_allocation_before_the_client_repaints() {
+        let scale = layer_content_scale((1_000, 40).into(), (1_600, 40).into());
+
+        assert_eq!(scale, Scale::from((1.6, 1.0)));
+    }
+
+    #[test]
+    fn layer_content_scale_ignores_empty_allocations() {
+        assert_eq!(
+            layer_content_scale((0, 40).into(), (1_600, 40).into()),
+            Scale::from(1.0)
+        );
+    }
+
     #[test]
     fn clip_rect_matches_smithay_gles_projection() {
         let geometry = Rectangle::<i32, Physical>::new((10, 5).into(), (30, 40).into());