Fix for #498. Recalculate the things that use screen coordinates (delete area and

screen transformation matrix) when a scroll happened.
This is not done using a scroll handler since the update
is expensive (getScreenCTM and getClientBoundingRect) and we don't need to do it
until the scroll is done and the user is interacting with blocks again.

core/block_svg.js

@@ -533,6 +533,7 @@ Blockly.BlockSvg.prototype.onMouseDown_ = function(e) {
   if (this.isInFlyout) {
     return;
   }
+  this.workspace.updateScreenCalculationsIfScrolled();
   this.workspace.markFocused();
   Blockly.terminateDrag_();
   this.select();

core/workspace_svg.js

@@ -147,6 +147,15 @@ Blockly.WorkspaceSvg.prototype.scrollbar = null;
  */
 Blockly.WorkspaceSvg.prototype.lastSound_ = null;
 
+/**
+ * Last known position of the page scroll.
+ * This is used to determine whether we have recalculated screen coordinate
+ * stuff since the page scrolled.
+ * @type {!goog.math.Coordinate}
+ * @private
+ */
+Blockly.WorkspaceSvg.prototype.lastRecordedPageScroll_ = null;
+
 /**
  * Inverted screen CTM, for use in mouseToSvg.
  * @type {SVGMatrix}
@@ -343,6 +352,16 @@ Blockly.WorkspaceSvg.prototype.addFlyout_ = function() {
   this.svgGroup_.insertBefore(svgFlyout, this.svgBlockCanvas_);
 };
 
+/**
+ * Update items that use screen coordinate calculations
+ * because something has changed (e.g. scroll position, window size).
+ * @private
+ */
+Blockly.WorkspaceSvg.prototype.updateScreenCalculations_ = function() {
+  this.updateInverseScreenCTM();
+  this.recordDeleteAreas();
+};
+
 /**
  * Resize the parts of the workspace that change when the workspace
  * contents (e.g. block positions) change.  This will also scroll the
@@ -382,11 +401,25 @@ Blockly.WorkspaceSvg.prototype.resize = function() {
   if (this.scrollbar) {
     this.scrollbar.resize();
   }
-
-  this.updateInverseScreenCTM();
-  this.recordDeleteAreas();
+  this.updateScreenCalculations_();
 };
 
+/**
+ * Resizes and repositions workspace chrome if the page has a new
+ * scroll position.
+ * @package
+ */
+Blockly.WorkspaceSvg.prototype.updateScreenCalculationsIfScrolled
+    = function() {
+  /* eslint-disable indent */
+  var currScroll = goog.dom.getDocumentScroll();
+  if (!goog.math.Coordinate.equals(this.lastRecordedPageScroll_,
+     currScroll)) {
+    this.lastRecordedPageScroll_ = currScroll;
+    this.updateScreenCalculations_();
+  }
+}; /* eslint-enable indent */
+
 /**
  * Get the SVG element that forms the drawing surface.
  * @return {!Element} SVG element.