There's a few things needed here:

• The <pane-splitter> element needs to be able to handle collapsed panes
• The 3-pane should respond to items on the view menu for showing/hiding the panes
• Pane state should be inherited by tabs opened from the current tab, or if that's not available from the XUL store

Several things happening here:

• Added a new pref mail.useNewMailTabs which determines if about:3pane/about:message is used when opening a folder or a message in a tab.
• In the shared command controller, added the ability to add more commands. about:3pane can now add commands that don't apply to about:message.
• Used commands to show/hide folder pane/message pane based on the View menu items. The menu's state doesn't yet reflect the current state of things.
• Collected the panes' collapsed state and size from the XUL store or from the previous tab.

Depends on D133018

Gonna keep using this bug for more things.

This cleans up a few broken bits and pieces that stopped the features from working.

This code will (eventually) perform the actions on the View menu. Some of them will require changes to the menu itself, but those changes aren't included here, as they'd break things.

The globals gMessage and gMessageURI only have values if the message pane is open, but in most cases where they are referenced it's the currently selected message we're looking for, which is independent of the message pane state.

Also fixes some methods in nsMsgDBView and JsTreeSelection which give strange responses if no message is selected.

Depends on D135976

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/d198e6797d17
Stop referencing gMessage[URI] in about:3pane. r=mkmelin
https://hg.mozilla.org/comm-central/rev/922212c4ebd7
Move ConversationOpener to a module so about:3pane can use it. r=mkmelin

Many of these commands will be unusable until changes to the main window, which are not part of this patch, land.

Depends on D136099

This folder tree is only a placeholder but it needs to handle added and removed folders or the tests will never pass.

Also fixes and enables the test browser_paneFocus.js, which has been broken and disabled since E10s was switched on.
The test doesn't actually pass with about:3pane enabled yet for a few reasons, but at least it passes with about:3pane disabled.

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/878494ea5f23
Make the F6 key shift focus correctly in about:3pane. r=mkmelin