Limit canvas height in heap map.

runtime/bin/vmservice/client/lib/src/elements/heap_map.dart

Index: runtime/bin/vmservice/client/lib/src/elements/heap_map.dart
===================================================================
--- runtime/bin/vmservice/client/lib/src/elements/heap_map.dart	(revision 37317)
+++ runtime/bin/vmservice/client/lib/src/elements/heap_map.dart	(working copy)
@@ -61,6 +61,9 @@
   static final _freeColor = [255, 255, 255, 255];
   static final _pageSeparationColor = [0, 0, 0, 255];
   static const _PAGE_SEPARATION_HEIGHT = 4;
+  // Many browsers will not display a very tall canvas.
+  // TODO(koda): Improve interface for huge heaps.
+  static const _MAX_CANVAS_HEIGHT = 6000;
 
   @observable String status;
   @published ServiceMap fragmentation;
@@ -165,7 +168,7 @@
     _pageHeight = _PAGE_SEPARATION_HEIGHT +
         fragmentation['page_size_bytes'] ~/
         fragmentation['unit_size_bytes'] ~/ width;
-    var height = _pageHeight * pages.length;
+    var height = min(_pageHeight * pages.length, _MAX_CANVAS_HEIGHT);
     _fragmentationData =
         _fragmentationCanvas.context2D.createImageData(width, height);
     _fragmentationCanvas.width = _fragmentationData.width;
@@ -178,10 +181,11 @@
   void _renderPages(int startPage) {
     var pages = fragmentation['pages'];
     status = 'Loaded $startPage of ${pages.length} pages';
-    if (startPage >= pages.length) {
+    var startY = startPage * _pageHeight;
+    var endY = startY + _pageHeight;
+    if (startPage >= pages.length || endY > _fragmentationData.height) {
       return;
     }
-    var startY = startPage * _pageHeight;
     var pixel = new PixelReference(_fragmentationData, new Point(0, startY));
     var objects = pages[startPage]['objects'];
     for (var i = 0; i < objects.length; i += 2) {
@@ -193,7 +197,6 @@
         pixel = pixel.next();
       }
     }
-    var endY = startY + _pageHeight;
     while (pixel.point.y < endY) {
       pixel.color = _pageSeparationColor;
       pixel = pixel.next();