Bug 561979 - Scrolling with mouse dragging does not work when cursor is over span element with 'overflow:auto' style; r=arpad.borsos

toolkit/content/widgets/browser.xml

@@ -825,13 +825,15 @@
               // we already discarded non-multiline selects so allow vertical
               // scroll for multiline ones directly without checking for a
               // overflow property
-              var scrollVert = this._scrollable.scrollHeight > this._scrollable.clientHeight &&
+              var scrollVert = this._scrollable.clientHeight > 0 &&
+                               this._scrollable.scrollHeight > this._scrollable.clientHeight &&
                                (this._scrollable instanceof HTMLSelectElement ||
                                 scrollingAllowed.indexOf(overflowy) >= 0);
 
               // do not allow horizontal scrolling for select elements, it leads
               // to visual artifacts and is not the expected behavior anyway
               if (!(this._scrollable instanceof HTMLSelectElement) &&
+                  this._scrollable.clientWidth > 0 &&
                   this._scrollable.scrollWidth > this._scrollable.clientWidth &&
                   scrollingAllowed.indexOf(overflowx) >= 0) {
                 this._autoScrollPopup.setAttribute("scrolldir", scrollVert ? "NSEW" : "EW");