Custom MailMate (http://freron.com) layout for Mailboxes view. Extends the installed widescreen layout by adding a vertical thread arcs view at the right. See screen shot at https://www.dropbox.com/s/ujdz5mgifhpq8ke/mailmate-widescreen-threadarcs.png . Save it to `~/Library/Application Support/MailMate/Resources/Layouts/Mailboxes/widescreen_with…

widescreen_with_thread_arcs.plist

/*	Custom MailMate (http://freron.com) layout for Mailboxes view. Extends the installed widescreen 
	layout by adding a vertical thread arcs view at the right. Save it to 
	"~/Library/Application Support/MailMate/Resources/Layouts/Mailboxes/widescreen_with_thread_arcs.plist" 
	and it will be available after a restart of MailMate. */

{
  name = "Widescreen with Thread Arcs";
	rootViewer =
	{
		viewerType = "MmSplitView";
		orientation = "horizontal";
		children =
		(
			{
				viewerType = "MmBoxView";
				orientation = "vertical";
				defaultSize = 200;
				minWidth = 100;
				autoresize = 0;
				toggleMenuTitle = "Mailboxes";
				toggleKeyEquivalent = "^~@d";

				dragRect = { width = 16.0; height = 16.0; location = "bottomRight"; };

				children =
				(
					{
						identifier = "mailboxesOutline";
						viewerType = "MmMailboxesOutlineView";
						stickyMessages = 1;
					},
					{
						viewerType = "MmMailboxesStatusBar";
					},
				);
			},
			{
				viewerType = "MmBoxView";
				orientation = "vertical";
				children =
				(
					{
						identifier = "pathControl";
						viewerType = "MmPathControlView";
						sources = ( { sourceIdentifier = "mailboxesOutline"; } );
					},
					{	viewerType = "MmSeparatorView"; showWithViews = ( "pathControl" ); },
					{
						viewerType = "MmSplitView";
						collapsibleSubview = 2; // Incomplete support -- no way to get it back (1 = first, 2 = last)
						children =
						(
							{
								identifier = "mainOutline";
								viewerType = "MmMessagesOutlineView";
								columnSettings = "outlineColumns";
								sources = ( { sourceIdentifier = "pathControl"; } );
							},
							{
								inject = ( "messageView.plist" );
							},
							// additional vertical thread arcs view
							{
								viewerType = "MmThreadArcsView";
								sources = ( { sourceIdentifier = "mainOutline"; } );
								selectionSources = ( { sourceIdentifier = "mainOutline"; } );
								transformation = "$ALL_MESSAGES.filter(#thread-id =[c] $mainOutline.#thread-id)";
								defaultSize = 50;
								minHeight = 30;
								minWidth = 50;
								orientation = vertical;
								toolTipFormatting = 
								{
									children =
									(
										{ formatString = "${from.name:${from.address}}"; },
										{ prefixString = ", "; formatString = "${#date-received-relative}"; },
									);
								};
							},
						);
					},
				);
			},
		);
	};
}

With the latest MailMate version, I'm seeing that some of the icons (Reply, Reply all, Flag) are greyed out when using this layout.

Thanks for the comment, I've never actually noticed that! I never use those buttons, since I control almost everything via keybindings. Nevertheless I will try to fix it, if possible.

I could not solve the problem, but there is a ticket in the official bug tracker marked as "reproduced": http://freron.lighthouseapp.com/projects/58672/tickets/343-icons-greyed-out Let's see, what happens.

Thanks a lot for this.
And the problem re: greyed out buttons in the toolbars no longer occurs.