Bug 1893956 - Disallow underlay compositor surfaces during low-quality pinch zoom. r=gfx-reviewers,nical

Underlay compositor surfaces require a cutout to be rendered to the
picture cache tiles above the underlay. During low-quality pinch zoom
the screen position of the cutout after the picture cache tiles have
been composited may not align exactly with the position of the
underlay surface which is composited directly. This can result in
visible seams along the edges of video.

Additionally, in underlay mode picture tiles still get invalidated
during a low quality zoom, which results in worse performance.

This patch therefore disallows Underlay compositor surfaces whilst a
low-quality pinch zoom is active.

Differential Revision: https://phabricator.services.mozilla.com/D222638

Author: jamienicol

webrender/src/picture.rs

@@ -1878,6 +1878,7 @@ enum SurfacePromotionFailure {
     UnderlayAlphaBackdrop,
     UnderlaySurfaceLimit,
     UnderlayIntersectsOverlay,
+    UnderlayLowQualityZoom,
     NotRootTileCache,
     ComplexTransform,
     SliceAtomic,
@@ -1897,6 +1898,7 @@ impl Display for SurfacePromotionFailure {
                 SurfacePromotionFailure::UnderlayAlphaBackdrop => "underlay requires an opaque backdrop",
                 SurfacePromotionFailure::UnderlaySurfaceLimit => "hit the underlay surface limit",
                 SurfacePromotionFailure::UnderlayIntersectsOverlay => "underlay intersects already-promoted overlay",
+                SurfacePromotionFailure::UnderlayLowQualityZoom => "underlay not allowed during low-quality pinch zoom",
                 SurfacePromotionFailure::NotRootTileCache => "is not on a root tile cache",
                 SurfacePromotionFailure::ComplexTransform => "has a complex transform",
                 SurfacePromotionFailure::SliceAtomic => "slice is atomic",
@@ -2559,6 +2561,15 @@ impl TileCacheInstance {
                 if self.overlay_region.intersects(&pic_coverage_rect) {
                     return Err(UnderlayIntersectsOverlay);
                 }
+
+                // Underlay cutouts are difficult to align with compositor surfaces when
+                // compositing during low-quality zoom, and the required invalidation
+                // whilst zooming would prevent low-quality zoom from working efficiently.
+                if frame_context.config.low_quality_pinch_zoom &&
+                    frame_context.spatial_tree.get_spatial_node(prim_spatial_node_index).is_ancestor_or_self_zooming
+                {
+                    return Err(UnderlayLowQualityZoom);
+                }
             }
             CompositorSurfaceKind::Blit => unreachable!(),
         }