TB115: in single mail tab Go to next unread message - should jump to next unread folder if no unread mails left

[RealRaven2000]

In the old version of Thunderbird (102) clicking on the button "jump to next unread mail" went to the next unread mail until no unread mails were left. The next time it was clicked it focused the first Thunderbird tab and advanced to the next email there and closed the single message tab. In Thunderbird 115 this currently doesn't do anything.

This behavior is actually changed in Thunderbird itself. If possible, it would be nice to move on to the next unread folder from within the tab instead, so that the single message tab would always display the next unread message.

[RealRaven2000]

While I am fixing this I see that Thunderbird logs an error "tabmail is undefined" when we execute the built in command "cmd_nextUnreadMsg" - this also happens if we use the standard toolbar button.

It happens in mailCommon.js when executing the following line:

this._navigate(this._navigationCommands[command]);

log:

TypeError: tabmail is null ext-mailTabs.js:88:21
    handleEvent chrome://messenger/content/parent/ext-mailTabs.js:88
    OnStopRunningUrl chrome://messenger/content/aboutMessage.js:218
    enter resource://devtools/server/actors/utils/event-loop.js:82
    _pauseAndRespond resource://devtools/server/actors/thread.js:953
    pauseAndRespond resource://devtools/server/actors/thread.js:1141
    _makeOnStep resource://devtools/server/actors/thread.js:1076
    createStepForReactionTracking resource://devtools/server/actors/thread.js:96
    doCommand chrome://messenger/content/mailCommon.js:657
    goDoCommand chrome://messenger/content/globalOverlay.js:99
    oncommand chrome://messenger/content/messageWindow.xhtml:1