Merge pull request #1243 from abarth/edge_trigger

Stocks has both tabs reified in the wiget tree

packages/flutter/lib/src/widgets/pageable_list.dart

@@ -225,10 +225,10 @@ class _PageViewportElement extends VirtualViewportElement<PageViewport> {
     return -(widget.startOffset - startOffsetBase) * _containerExtent;
   }
 
-  void updateRenderObject() {
+  void updateRenderObject(PageViewport oldWidget) {
     renderObject.scrollDirection = widget.scrollDirection;
     renderObject.overlayPainter = widget.overlayPainter;
-    super.updateRenderObject();
+    super.updateRenderObject(oldWidget);
   }
 
   double _containerExtent;

packages/flutter/lib/src/widgets/scrollable_grid.dart

@@ -122,9 +122,9 @@ class _GridViewportElement extends VirtualViewportElement<GridViewport> {
   double get startOffsetLimit =>_startOffsetLimit;
   double _startOffsetLimit;
 
-  void updateRenderObject() {
+  void updateRenderObject(GridViewport oldWidget) {
     renderObject.delegate = widget.delegate;
-    super.updateRenderObject();
+    super.updateRenderObject(oldWidget);
   }
 
   double _contentExtent;

packages/flutter/lib/src/widgets/scrollable_list.dart

@@ -135,12 +135,12 @@ class _ListViewportElement extends VirtualViewportElement<ListViewport> {
   double get startOffsetLimit =>_startOffsetLimit;
   double _startOffsetLimit;
 
-  void updateRenderObject() {
+  void updateRenderObject(ListViewport oldWidget) {
     renderObject.scrollDirection = widget.scrollDirection;
     renderObject.itemExtent = widget.itemExtent;
     renderObject.padding = widget.padding;
     renderObject.overlayPainter = widget.overlayPainter;
-    super.updateRenderObject();
+    super.updateRenderObject(oldWidget);
   }
 
   double _contentExtent;

packages/flutter/lib/src/widgets/virtual_viewport.dart

@@ -43,7 +43,7 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
     _iterator = null;
     _widgets = <Widget>[];
     renderObject.callback = layout;
-    updateRenderObject();
+    updateRenderObject(null);
   }
 
   void unmount() {
@@ -56,8 +56,9 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
       _iterator = null;
       _widgets = <Widget>[];
     }
+    T oldWidget = widget;
     super.update(newWidget);
-    updateRenderObject();
+    updateRenderObject(oldWidget);
     if (!renderObject.needsLayout)
       _materializeChildren();
   }
@@ -73,7 +74,7 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
     }
   }
 
-  void updateRenderObject() {
+  void updateRenderObject(T oldWidget) {
     renderObject.virtualChildCount = widget.children.length;
 
     if (startOffsetBase != null) {
@@ -82,9 +83,22 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
       // If we don't already need layout, we need to request a layout if the
       // viewport has shifted to expose new children.
       if (!renderObject.needsLayout) {
-        if (startOffsetBase != null && widget.startOffset < startOffsetBase)
-          renderObject.markNeedsLayout();
-        else if (startOffsetLimit != null && widget.startOffset > startOffsetLimit)
+        bool shouldLayout = false;
+        if (startOffsetBase != null) {
+          if (widget.startOffset < startOffsetBase)
+            shouldLayout = true;
+          else if (widget.startOffset == startOffsetBase && oldWidget?.startOffset != startOffsetBase)
+            shouldLayout = true;
+        }
+
+        if (startOffsetLimit != null) {
+          if (widget.startOffset > startOffsetLimit)
+            shouldLayout = true;
+          else if (widget.startOffset == startOffsetLimit && oldWidget?.startOffset != startOffsetLimit)
+            shouldLayout = true;
+        }
+
+        if (shouldLayout)
           renderObject.markNeedsLayout();
       }
     }