Added comments & assert to scrollX fix.

Added comments to fix as per @kripken's request.
Added an assert too (guarded by ASSERTIONS define)

1 files changed, 9 insertions, 0 deletions

diff --git a/src/library_browser.js b/src/library_browser.js
index f3363e51..bf8444bd 100644
--- a/src/library_browser.js
+++ b/src/library_browser.js
@@ -478,8 +478,17 @@ mergeInto(LibraryManager.library, {
         // in the coordinates.
         var rect = Module["canvas"].getBoundingClientRect();
         var x, y;
+        
+        //Neither .scrollX or .pageXOffset are defined in a spec, but
+        //we prefer .scrollX because it is currently in a spec draft.
+        //(see: http://www.w3.org/TR/2013/WD-cssom-view-20131217/)
         var scrollX = ((window.scrollX !== undefined) ? window.scrollX : window.pageXOffset);
         var scrollY = ((window.scrollY !== undefined) ? window.scrollY : window.pageYOffset);
+        #if ASSERTIONS
+        //If this assert lands, it's likely because the browser doesn't support scrollX or pageXOffset
+        //and we have no viable fallback.
+        assert((scrollX !== undefined) && (scrollY !== undefined), 'Unable to retrieve scroll position, mouse positions likely broken.');
+        #endif
         if (event.type == 'touchstart' ||
             event.type == 'touchend' ||
             event.type == 'touchmove') {