fix for #77506. only fire this handler when the grippy is clicked.

if we fire it when the splitter is clicked, we'll often stop
displaying the message in the message pane.
r=bhuvan,sr=mscott

mailnews/base/resources/content/mail3PaneWindowVertLayout.xul

@@ -154,8 +154,8 @@ Rights Reserved.
       </box>
       <!-- if you change this id, please change GetThreadAndMessagePaneSplitter() and MsgToggleMessagePane() -->
       <splitter id="threadpane-splitter" collapse="after" persist="state"
-         onclick="OnClickThreadAndMessagePaneSplitter()" orient="vertical" autostretch="never">
-        <grippy/>
+         orient="vertical" autostretch="never">
+        <grippy onclick="OnClickThreadAndMessagePaneSplitter()" />
       </splitter>
       <!-- msg header view -->
       <box id="messagepanebox" orient="vertical" flex="4" persist="collapsed height" class="window-focusborder" focusring="false">

mailnews/base/resources/content/messenger.xul

@@ -155,9 +155,9 @@ Rights Reserved.
       <outliner id="threadOutliner" flex="2" persist="height" style="height:0px" context="threadPaneContext"/>
 
       <!-- if you change this id, please change GetThreadAndMessagePaneSplitter() and MsgToggleMessagePane() -->
-      <splitter collapse="after" persist="state" onclick="OnClickThreadAndMessagePaneSplitter()"
+      <splitter collapse="after" persist="state" 
                 id="threadpane-splitter" orient="vertical" autostretch="never">
-        <grippy/>
+        <grippy onclick="OnClickThreadAndMessagePaneSplitter()"/>
       </splitter>
       
       <box id="messagepanebox" align="vertical" flex="3" persist="collapsed height" class="window-focusborder" focusring="false" onclick="contentAreaClick(event);" ondraggesture="nsDragAndDrop.startDrag(event, contentAreaDNDObserver);">