Try to find a scrollable element rather than requiring a click when one has not been selected. Closes issue #907.

common/modules/base.jsm

@@ -217,10 +217,37 @@ function properties(obj, prototypes, debugger_) {
     }
     catch (e) {}
 
+    function props(obj) {
+        // Grr.
+        try {
+            return Object.getOwnPropertyNames(obj);
+        }
+        catch (e) {
+            if (e.result === Cr.NS_ERROR_FAILURE) {
+                // This is being thrown for PDF.js content windows,
+                // currently.
+                let filter = function filter(prop) {
+                    try {
+                        return k in obj;
+                    }
+                    catch (e) {}
+                    return false;
+                };
+                return array.uniq([k for (k in obj)].concat(
+                    Object.getOwnPropertyNames(
+                              XPCNativeWrapper.unwrap(obj))
+                          .filter(filter)))
+            }
+            else if (!e.stack) {
+                throw Error(e);
+            }
+        }
+    }
+
     for (; obj; obj = prototypes && prototype(obj)) {
         try {
-            if (sandbox.Object.getOwnPropertyNames || !debugger_ || !services.debugger.isOn)
-                var iter = (v for each (v in Object.getOwnPropertyNames(obj)));
+            if (!debugger_ || !services.debugger.isOn)
+                var iter = (v for each (v in props(obj)));
         }
         catch (e) {}
         if (!iter)

common/modules/buffer.jsm

@@ -787,8 +787,26 @@ var Buffer = Module("Buffer", {
             var sel = this.focusedFrame.getSelection();
         }
         catch (e) {}
+
         if (!elem && sel && sel.rangeCount)
             elem = sel.getRangeAt(0).startContainer;
+
+        if (!elem) {
+            let area = -1;
+            for (let e in DOM(Buffer.SCROLLABLE_SEARCH_SELECTOR,
+                              this.focusedFrame.document)) {
+                if (Buffer.isScrollable(e, dir, horizontal)) {
+                    let r = DOM(e).rect;
+                    let a = r.width * r.height;
+                    if (a > area) {
+                        area = a;
+                        elem = e;
+                    }
+                }
+            }
+            if (elem)
+                util.trapErrors("focus", elem);
+        }
         if (elem)
             elem = find(elem);
 
@@ -1255,6 +1273,12 @@ var Buffer = Module("Buffer", {
     scrollTo: deprecated("Buffer.scrollTo", function scrollTo(x, y) this.win.scrollTo(x, y)),
     textZoom: deprecated("buffer.zoomValue/buffer.fullZoom", function textZoom() this.contentViewer.markupDocumentViewer.textZoom * 100)
 }, {
+    /**
+     * The pattern used to search for a scrollable element when we have
+     * no starting point.
+     */
+    SCROLLABLE_SEARCH_SELECTOR: "div",
+
     PageInfo: Struct("PageInfo", "name", "title", "action")
                         .localize("title"),