Bug #215661 --> add a new hidden layout format that stretches the thread pane aross the top with the folder and message pane side by side on the bottom. See the bug for caveats and instructions for testing this new layout.

mail/base/content/mailWindowOverlay.xul

@@ -1199,6 +1199,10 @@
                         accesskey="&messagePaneWide.accesskey;" oncommand="ChangeMailLayout(kWideMailLayout);"/>
               <menuitem id="messagePaneVertical" type="radio" label="&messagePaneVertical.label;" name="viewlayoutgroup"
                         accesskey="&messagePaneVertical.accesskey;" oncommand="ChangeMailLayout(kVerticalMailLayout);"/>
+<!--
+              <menuitem id="threadPaneWide" type="radio" label="&threadPaneWide.label;" name="viewlayoutgroup"
+                        accesskey="&threadPaneWide.accesskey;" oncommand="ChangeMailLayout(kWideThreadPaneConfig);"/>
+-->
               <menuseparator/>
               <menuitem id="menu_showMessage" type="checkbox" label="&showMessageCmd.label;" key="key_toggleMessagePane"
                         accesskey="&showMessageCmd.accesskey;" oncommand="MsgToggleMessagePane();"/>

mail/base/content/msgMail3PaneWindow.js

@@ -51,6 +51,7 @@ const kMailCheckOncePrefName = "mail.startup.enabledMailCheckOnce";
 const kStandardPaneConfig = 0;
 const kWidePaneConfig = 1;
 const kVerticalPaneConfig = 2;
+const kWideThreadPaneConfig = 3;
 
 const kNumFolderViews = 4; // total number of folder views
 
@@ -671,21 +672,20 @@ function UpdateMailPaneConfig(aMsgWindowInitialized) {
     document.getElementById('messagepanebox').setAttribute('flex', 1);
   else 
     document.getElementById('messagepanebox').removeAttribute('flex');
-    
-  
+      
   // don't do anything if we are already in the correct configuration
   if (paneConfig == gCurrentPaneConfig)
     return;
 
   var mailContentWrapper = document.getElementById("mailContentWrapper");
   var messagesBox = document.getElementById("messagesBox");
+  var messengerBox = document.getElementById("messengerBox");
   var messagePaneBox = GetMessagePane();
   var msgPaneReRooted = false;
-
   var threadPaneSplitter = GetThreadAndMessagePaneSplitter();
 
   // the only element we need to re-root is the message pane.
-  var desiredMsgPaneParentId = (paneConfig == "0" || paneConfig == "2") ? "messagesBox" : "mailContentWrapper";
+  var desiredMsgPaneParentId = (paneConfig == "0" || paneConfig == "2" || paneConfig == "3") ? "messagesBox" : "mailContentWrapper";
 
   if (messagePaneBox.parentNode.id != desiredMsgPaneParentId)
   {
@@ -698,6 +698,20 @@ function UpdateMailPaneConfig(aMsgWindowInitialized) {
      messagePaneNewParent.appendChild(messagePaneBox); 
      msgPaneReRooted = true;
   }
+  
+  /* this code doesn't work yet, see the comment below about kWideThreadPaneConfig
+  if (gCurrentPaneConfig == kWideThreadPaneConfig)
+  {   
+    threadPaneSplitter.setAttribute("orient", "vertical");
+    mailContentWrapper.setAttribute("orient", "horizontal");
+    mailContentWrapper.removeChild(threadPaneSplitter);
+    mailContentWrapper.removeChild(messagePaneBox);
+    messagesBox.insertBefore(threadPaneSplitter, messagesBox.firstChild);
+    messagesBox.insertBefore(messengerBox, messagesBox.firstChild);
+    messagePaneBox.removeAttribute("flex");
+    msgPaneReRooted = true;
+  }
+  */
 
   // now for each config, handle any extra clean up to create that view (such as changing a box orientation)
   if (paneConfig == kStandardPaneConfig) // standard 3-Pane Layout
@@ -708,7 +722,6 @@ function UpdateMailPaneConfig(aMsgWindowInitialized) {
     mailContentWrapper.setAttribute("orient", "horizontal");
     messagesBox.setAttribute("orient", "vertical");
   }
-
   else if (paneConfig == kWidePaneConfig)  // "Wide" Window Pane Layout
   {     
     threadPaneSplitter.setAttribute("orient", "vertical");
@@ -716,8 +729,7 @@ function UpdateMailPaneConfig(aMsgWindowInitialized) {
     // finally, make sure mailContentWrapper has the correct orientation
     mailContentWrapper.setAttribute("orient", "vertical");
     messagesBox.setAttribute("orient", "vertical");
-  }
-  
+  }  
   else if (paneConfig == kVerticalPaneConfig) // Vertical Pane Layout
   {
     messagesBox.setAttribute("orient", "horizontal");
@@ -725,6 +737,22 @@ function UpdateMailPaneConfig(aMsgWindowInitialized) {
     // finally, make sure mailContentWrapper has the correct orientation
     mailContentWrapper.setAttribute("orient", "horizontal");
   }
+  else if (paneConfig == kWideThreadPaneConfig) 
+  {
+    // kWideThreadPaneConfig is a easter egg layout which isn't fully polished. So 
+    // the menu item for selecting it is hidden from the UI.
+    // If you change from kWideThreadPaneConfig to another layout, you have to restart
+    // Thunderbird before things look right. Loading account central looks really bad.
+    // When you change into this layout, the thread pane gets re-rooted and we don't
+    // handle that properly, the user must re-select the folder before the thread pane
+    // relists the messages in it.
+    mailContentWrapper.insertBefore(threadPaneSplitter, mailContentWrapper.firstChild);
+    mailContentWrapper.insertBefore(messengerBox, mailContentWrapper.firstChild);
+
+    mailContentWrapper.setAttribute("orient", "vertical");
+    threadPaneSplitter.setAttribute("orient", "vertical");
+    messagePaneBox.setAttribute("flex", "1");
+  }
 
   // re-rooting the message pane causes the docshell to get destroyed 
   // and replaced with another one. As such, we need to re-set the window (and thus the 

mail/locales/en-US/chrome/messenger/messenger.dtd

@@ -113,6 +113,8 @@
 <!ENTITY messagePaneWide.accesskey "W">
 <!ENTITY messagePaneVertical.label "Vertical View">
 <!ENTITY messagePaneVertical.accesskey "V">
+<!ENTITY threadPaneWide.label "Wide List View">
+<!ENTITY threadPaneWide.accesskey "i">
 <!ENTITY showMessageCmd.label "Message Pane">
 <!ENTITY showMessageCmd.accesskey "M">