Accessing the Thunderbird message when closing a message tab / window - xul

Question

We have developed a Thunderbird (11) plugin that allows us to save the content of a message to disk. Now we are extending this extension to allow automatic processing of a message when you close it. We run into a number of issues:
We cannot find a way to hook into a 'close tab' event. We are also having trouble getting the Message URI of the currently open tabs (we are trying catching click and keyboard events now). This information does not appear to be available in the DOM of the tab container.
Is there a way to detect closing of a mail message tab or window in a generic way, together with retrieving the URI of the closed mail message for further processing?
We have looked at the documentation of the tab container, the NsIWindowMediator, tried various event listeners, but no luck so far.
Edit: We are getting some results using the most recently closed tabs list. Not a very elegant solution but at least we have a reference to the tab. Now we only have to get the URI to the message that was contained inside the tab.

Answer 1

We cannot find a way to hook into a 'close tab' event.
The (badly documented) <tabmail> element allows registering tab monitors. Something like this should work:
var tabmail = document.getElementById("tabmail");
var monitor = {
onTabClosing: function(tab)
{
...
}
};
tabmail.registerTabMonitor(monitor);
We are also having trouble getting the Message URI of the currently open tabs
The <tabmail> element has a property tabInfo containing information on the currently open tabs. You probably want to look only at the tabs where mode.name is "message" (there is a bunch of other modes as well, e.g. "folder" or "contentTab"). This mode has a getBrowser() method, so something like this should do:
var tabmail = document.getElementById("tabmail");
for (var i = 0; i < tabmail.tabInfo.length; i++)
{
var tab = tabmail.tabInfo[i];
if (tab.mode.name == "message")
alert(tab.mode.getBrowser().currentURI.spec);
}
Edit: As Peter points out in the comments, the approach to get the URI for a message will only work the currently loaded message - all tabs reuse the same browser element for the mail messages. Getting the URI properly is more complicated, you have to get the nsIMsgDBHdr instance for the message via TabInfo.folderDisplay.selectedMessage and then use nsIMsgFolder.getUriForMsg() to construct the URI for it:
var tabmail = document.getElementById("tabmail");
for (var i = 0; i < tabmail.tabInfo.length; i++)
{
var tab = tabmail.tabInfo[i];
if (tab.mode.name != "message")
continue;
var message = tab.folderDisplay.selectedMessage;
alert(message.folder.getUriForMsg(message));
}

Answer 2

For the second part of the question:
The following example code will at provide you the msgDBHdr objects of all opened tabs. You should do some checks on the type to avoid accessing a message in a calendar tab.):
tabInfos = window.document.getElementById("tabmail").tabInfo;
for (i = 0; i < tabInfos.length; i++) {
msgHdr = tabInfos[i].folderDisplay.selectedMessage;
alert(
msgHdr.mime2DecodedSubject+"\n"
+msgHdr.messageId+"\n"
+"in view type "+tabInfos[i].mode.type
);
}
The tabinfo entries have some further interesting information. Just open the ErrorConsole and run
top.opener.window.document.getElementById("tabmail").tabInfo[0].toSource()
and read through it carefully.