From 244733467de0ca92dd667afab5a9aa1aacfcad70 Mon Sep 17 00:00:00 2001
From: Katelyn Mann <picklesrus@users.noreply.github.com>
Date: Wed, 10 Aug 2016 18:00:15 -0700
Subject: [PATCH] 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     |  1 +
 core/workspace_svg.js | 39 ++++++++++++++++++++++++++++++++++++---
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/core/block_svg.js b/core/block_svg.js
index 7369235d5..719c967e5 100644
--- a/core/block_svg.js
+++ b/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();
diff --git a/core/workspace_svg.js b/core/workspace_svg.js
index 8e06fb415..1ce23e54f 100644
--- a/core/workspace_svg.js
+++ b/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.