fix for #73156. enter should work like open in the thread / search pane.

patch by the bossman (putterman@netscape.com), sr=sspitzer

mailnews/base/resources/content/threadPane.js

@@ -139,7 +139,8 @@ function ThreadPaneDoubleClick()
 
 function ThreadPaneKeyPress(event)
 {
-  return;
+    if (event.keyCode == 13)
+      ThreadPaneDoubleClick();
 }
 
 function MsgSortByDate()

mailnews/base/resources/content/threadPane.xul

@@ -34,7 +34,8 @@ Rights Reserved.
 <outliner id="threadOutliner" flex="1" 
     ondraggesture="return BeginDragThreadPane(event);"
     ondragdrop="return DropOnThreadPane(event);" 
-    onclick="ThreadPaneOnClick(event);">
+    onclick="ThreadPaneOnClick(event);"
+    onkeypress="ThreadPaneKeyPress(event);">
   <outlinercol id="threadCol" display="&threadColumn.label;" class="outlinercol-header outlinercol-image outlinercol-inset-header threadColumnHeader" currentView="unthreaded" cycler="true" />
   <outlinercol id="subjectCol" class="outlinercol-header outlinercell-inset-header sortDirectionIndicator" persist="hidden width" flex="7" label="&subjectColumn.label;" primary="true"/>
   <splitter class="tree-splitter"/>