Audit use of saveLayer (#11442)

It has recently come to light that we may have been ordering saveLayer
calls and clip calls incorrectly.

packages/flutter/lib/src/material/time_picker.dart

@@ -429,7 +429,7 @@ class _DialPainter extends CustomPainter {
       center: focusedPoint, radius: focusedRadius
     );
     canvas
-      ..saveLayer(focusedRect, new Paint())
+      ..save()
       ..clipPath(new Path()..addOval(focusedRect));
     paintLabels(secondaryLabels);
     canvas.restore();

packages/flutter/lib/src/painting/flutter_logo.dart

@@ -466,6 +466,7 @@ class _FlutterLogoPainter extends BoxPainter {
         final double fontSize = 0.35 * logoTargetSquare.height * (1 - (10.4 * 2.0) / 202.0);
         final double scale = fontSize / 100.0;
         if (_config._position > -1.0) {
+          // This limits what the drawRect call below is going to blend with.
           canvas.saveLayer(_textBoundingRect, new Paint());
         } else {
           canvas.save();

packages/flutter/lib/src/rendering/object.dart

@@ -330,10 +330,12 @@ class PaintingContext {
     if (needsCompositing) {
       pushLayer(new ClipRectLayer(clipRect: offsetClipRect), painter, offset, childPaintBounds: offsetClipRect);
     } else {
-      canvas.save();
-      canvas.clipRect(offsetClipRect);
+      canvas
+        ..save()
+        ..clipRect(offsetClipRect);
       painter(this, offset);
-      canvas.restore();
+      canvas
+        ..restore();
     }
   }
 
@@ -355,10 +357,14 @@ class PaintingContext {
     if (needsCompositing) {
       pushLayer(new ClipRRectLayer(clipRRect: offsetClipRRect), painter, offset, childPaintBounds: offsetBounds);
     } else {
-      canvas.saveLayer(offsetBounds, _defaultPaint);
-      canvas.clipRRect(offsetClipRRect);
+      canvas
+        ..save()
+        ..clipRRect(offsetClipRRect)
+        ..saveLayer(offsetBounds, _defaultPaint);
       painter(this, offset);
-      canvas.restore();
+      canvas
+        ..restore()
+        ..restore();
     }
   }
 
@@ -380,10 +386,14 @@ class PaintingContext {
     if (needsCompositing) {
       pushLayer(new ClipPathLayer(clipPath: offsetClipPath), painter, offset, childPaintBounds: offsetBounds);
     } else {
-      canvas.saveLayer(bounds.shift(offset), _defaultPaint);
-      canvas.clipPath(clipPath.shift(offset));
+      canvas
+        ..save()
+        ..clipPath(clipPath.shift(offset))
+        ..saveLayer(bounds.shift(offset), _defaultPaint);
       painter(this, offset);
-      canvas.restore();
+      canvas
+        ..restore()
+        ..restore();
     }
   }
 
@@ -407,10 +417,12 @@ class PaintingContext {
         childPaintBounds: MatrixUtils.inverseTransformRect(effectiveTransform, canvasBounds),
       );
     } else {
-      canvas.save();
-      canvas.transform(effectiveTransform.storage);
+      canvas
+        ..save()
+        ..transform(effectiveTransform.storage);
       painter(this, offset);
-      canvas.restore();
+      canvas
+        ..restore();
     }
   }
 

packages/flutter/lib/src/rendering/paragraph.dart

@@ -297,10 +297,13 @@ class RenderParagraph extends RenderBox {
 
     if (_hasVisualOverflow) {
       final Rect bounds = offset & size;
-      if (_overflowShader != null)
+      if (_overflowShader != null) {
+        // This layer limits what the shader below blends with to be just the text
+        // (as opposed to the text and its background).
         canvas.saveLayer(bounds, new Paint());
-      else
+      } else {
         canvas.save();
+      }
       canvas.clipRect(bounds);
     }
     _textPainter.paint(canvas, offset);

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -1351,13 +1351,19 @@ class RenderPhysicalModel extends _RenderCustomClip<RRect> {
           );
         }
         canvas.drawRRect(offsetClipRRect, new Paint()..color = color);
-        if (offsetClipRRect.isRect) {
-          canvas.save();
-        } else {
-          canvas.saveLayer(offsetBounds, _defaultPaint);
-        }
+        canvas.save();
         canvas.clipRRect(offsetClipRRect);
+        // We only use a new layer for non-rectangular clips, on the basis that
+        // rectangular clips won't need antialiasing. This is not really
+        // correct, because if we're e.g. rotated, rectangles will also be
+        // aliased. Unfortunately, it's too much of a performance win to err on
+        // the side of correctness here.
+        // TODO(ianh): Find a better solution.
+        if (!offsetClipRRect.isRect)
+          canvas.saveLayer(offsetBounds, _defaultPaint);
         super.paint(context, offset);
+        if (!offsetClipRRect.isRect)
+          canvas.restore();
         canvas.restore();
         assert(context.canvas == canvas, 'canvas changed even though needsCompositing was false');
       }