[Impeller] Fix back-to-back clip restoration (#38964)

Author: bdero

impeller/aiks/aiks_unittests.cc

@@ -270,6 +270,29 @@ TEST_P(AiksTest, CanRenderDifferenceClips) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, CanRenderWithContiguousClipRestores) {
+  Canvas canvas;
+
+  // Cover the whole canvas with red.
+  canvas.DrawPaint({.color = Color::Red()});
+
+  canvas.Save();
+
+  // Append two clips. First with empty coverage.
+  canvas.ClipPath(
+      PathBuilder{}.AddRect(Rect::MakeXYWH(100, 100, 100, 100)).TakePath());
+  canvas.ClipPath(
+      PathBuilder{}.AddRect(Rect::MakeXYWH(100, 100, 100, 100)).TakePath());
+
+  // Restore to no clips.
+  canvas.Restore();
+
+  // Replace the whole canvas with green.
+  canvas.DrawPaint({.color = Color::Green()});
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 TEST_P(AiksTest, ClipsUseCurrentTransform) {
   std::array<Color, 5> colors = {Color::White(), Color::Black(),
                                  Color::SkyBlue(), Color::Red(),

impeller/entity/entity_pass.cc

@@ -456,11 +456,14 @@ bool EntityPass::OnRender(
           return true;
         }
 
-        FML_DCHECK(stencil_stack.size() > 1);
+        auto restoration_depth =
+            element_entity.GetStencilDepth() - stencil_depth_floor;
+        FML_DCHECK(restoration_depth < stencil_stack.size());
 
-        stencil_stack.pop_back();
+        auto restored_coverage = stencil_stack.back().coverage;
+        stencil_stack.resize(restoration_depth + 1);
 
-        if (!stencil_stack.back().coverage.has_value()) {
+        if (!restored_coverage.has_value()) {
           // Running this restore op won't make anything renderable, so skip it.
           return true;
         }