[Impeller] better handle allocation herustics of Android slide in page transition.

impeller/entity/save_layer_utils.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "impeller/entity/save_layer_utils.h"
+#include "impeller/geometry/scalar.h"
 
 namespace impeller {
 
@@ -106,8 +107,34 @@ std::optional<Rect> ComputeSaveLayerCoverage(
 
   // Transform the input coverage into the global coordinate space before
   // computing the bounds limit intersection.
-  return coverage.TransformBounds(effect_transform)
-      .Intersection(coverage_limit);
+  Rect transformed_coverage = coverage.TransformBounds(effect_transform);
+  std::optional<Rect> intersection =
+      transformed_coverage.Intersection(coverage_limit);
+  if (!intersection.has_value()) {
+    return std::nullopt;
+  }
+
+  // Sometimes a saveLayer is only slightly shifted outside of the cull rect,
+  // but is being animated in. This is common for the Android slide in page
+  // transitions. In these cases, computing a cull that is too tight can cause
+  // thrasing of the texture cache. Instead, we try to determine the
+  // intersection using only the sizing by shifting the coverage rect into the
+  // cull rect origin.
+  Point delta = coverage_limit.GetOrigin() - transformed_coverage.GetOrigin();
+  if (ScalarNearlyEqual(delta.y, 0) || ScalarNearlyEqual(delta.x, 0)) {
+    Scalar threshold = std::max(std::abs(delta.x / coverage_limit.GetWidth()),
+                                std::abs(delta.y / coverage_limit.GetHeight()));
+    if (threshold < 0.3) {
+      std::optional<Rect> shifted_intersected_value =
+          transformed_coverage.Shift(delta).Intersection(coverage_limit);
+
+      if (shifted_intersected_value.has_value()) {
+        return shifted_intersected_value.value().Shift(-delta);
+      }
+    }
+  }
+
+  return intersection;
 }
 
 }  // namespace impeller

impeller/entity/save_layer_utils_unittests.cc

@@ -280,6 +280,49 @@ TEST(SaveLayerUtilsTest,
   EXPECT_EQ(coverage.value(), Rect::MakeLTRB(0, 0, 50, 50));
 }
 
+TEST(SaveLayerUtilsTest,
+     PartiallyIntersectingCoverageIgnoresOriginWithSlideSemanticsX) {
+  // X varies, translation is performed on coverage.
+  auto coverage = ComputeSaveLayerCoverage(
+      /*content_coverage=*/Rect::MakeLTRB(5, 0, 210, 210),  //
+      /*effect_transform=*/{},                              //
+      /*coverage_limit=*/Rect::MakeLTRB(0, 0, 100, 100),    //
+      /*image_filter=*/nullptr                              //
+  );
+
+  ASSERT_TRUE(coverage.has_value());
+  // Size that matches coverage limit
+  EXPECT_EQ(coverage.value(), Rect::MakeLTRB(5, 0, 105, 100));
+}
+
+TEST(SaveLayerUtilsTest,
+     PartiallyIntersectingCoverageIgnoresOriginWithSlideSemanticsY) {
+  // Y varies, translation is performed on coverage.
+  auto coverage = ComputeSaveLayerCoverage(
+      /*content_coverage=*/Rect::MakeLTRB(0, 5, 210, 210),  //
+      /*effect_transform=*/{},                              //
+      /*coverage_limit=*/Rect::MakeLTRB(0, 0, 100, 100),    //
+      /*image_filter=*/nullptr                              //
+  );
+
+  ASSERT_TRUE(coverage.has_value());
+  // Size that matches coverage limit
+  EXPECT_EQ(coverage.value(), Rect::MakeLTRB(0, 5, 100, 105));
+}
+
+TEST(SaveLayerUtilsTest, PartiallyIntersectingCoverageIgnoresOriginBothXAndY) {
+  // Both X and Y vary, no transation is performed.
+  auto coverage = ComputeSaveLayerCoverage(
+      /*content_coverage=*/Rect::MakeLTRB(5, 5, 210, 210),  //
+      /*effect_transform=*/{},                              //
+      /*coverage_limit=*/Rect::MakeLTRB(0, 0, 100, 100),    //
+      /*image_filter=*/nullptr                              //
+  );
+
+  ASSERT_TRUE(coverage.has_value());
+  EXPECT_EQ(coverage.value(), Rect::MakeLTRB(5, 5, 100, 100));
+}
+
 }  // namespace testing
 }  // namespace impeller