Bug 518545: middle click on scrollbar opens selected message in a new tab; r=Neil

suite/mailnews/msgMail3PaneWindow.js

@@ -1217,6 +1217,10 @@ function GetSelectedFolderIndex()
 // It will also keep the outline/dotted line in the original row.
 function ChangeSelectionWithoutContentLoad(event, tree)
 {
+  // usually, we're only interested in tree content clicks, not scrollbars etc.
+  if (event.originalTarget.localName != "treechildren")
+    return;
+
     var treeBoxObj = tree.treeBoxObject;
     var treeSelection = tree.view.selection;
 
@@ -1255,6 +1259,10 @@ function TreeOnMouseDown(event)
 
 function FolderPaneOnClick(event)
 {
+  // usually, we're only interested in tree content clicks, not scrollbars etc.
+  if (event.originalTarget.localName != "treechildren")
+    return;
+
   // we may want to open the folder in a new tab on middle click
   if (event.button == kMouseButtonMiddle)
   {
@@ -1281,11 +1289,6 @@ function FolderPaneOnClick(event)
       event.stopPropagation();
     }
   }
-  else if ((event.originalTarget.localName == "slider") ||
-           (event.originalTarget.localName == "scrollbarbutton"))
-  {
-    event.stopPropagation();
-  }
   else if ((event.detail == 2) &&
            (elt.value != "twisty") &&
            (folderTree.view.getLevel(row.value) != 0))

suite/mailnews/threadPane.js

@@ -42,10 +42,13 @@ var gThreadPaneCommandUpdater = null;
 
 function ThreadPaneOnClick(event)
 {
+  // usually, we're only interested in tree content clicks, not scrollbars etc.
+  let t = event.originalTarget;
+
   // we may want to open the message in a new tab on middle click
   if (event.button == kMouseButtonMiddle)
   {
-    if (AllowOpenTabOnMiddleClick())
+    if (t.localName == "treechildren" && AllowOpenTabOnMiddleClick())
     {
       // we don't allow new tabs in the search dialog
       if ("RestoreSelectionWithoutContentLoad" in window)
@@ -66,7 +69,6 @@ function ThreadPaneOnClick(event)
   // We also get in here for clicks on the "treecol" (headers) and the
   // "scrollbarbutton" (scrollbar buttons), but we don't want those events to
   // cause a "double click".
-  var t = event.originalTarget;
   if (t.localName == "treecol")
   {
     HandleColumnClick(t.id);