Allow add-ons to become "Tab Managers"

[Clozent]

I like using Sidebery to manage my tabs, because it has a vertical tree layout that expands automatically, among other features. Others like using TreeStyleTab for similar reasons, and I'm sure there are many other add-ons that manage tabs.

While generally okay, using such add-ons currently has many drawbacks:

1. If the add-on uses the sidebar, you might not be able to fully utilize the sidebar, since you need to keep the tab manager displayed.
2. You cannot hide the native tab bar, so you will have to either accept having tabs in two places instead of one, or delve into advanced configuration in order to hide the native tab bar (and also fix it occasionally when an update breaks it).
3. You cannot view a preview of the tab you are hovering over in the add-on (Sidebery works around this by embedding the preview in the current page, a solution that doesn't always work properly and might be used for fingerprinting).
4. Dragging-and-dropping tabs is heavily downgraded:
   • You can't drag-and-drop a tab outside of the browser window properly - it just opens it over the current window.
   • The new window is only created when dropping, instead of while dragging outside of the tab manager.
   • Similarly to the previous point, tabs are only moved into windows when they are dropped inside of the tab manager, instead of moving windows while dragging over the tab manager.
5. (This will not be relevant if drawback no. 1 is fixed) The sidebar cannot display in the "Customize Firefox" page, so to close it you have to either: use the native tab bar, use the keyboard shortcut to close the tab or somehow know that pressing the escape key also closes it.

I propose a "Tab Manager" permission (or, alternatively, a set of permissions) for add-ons, allowing them to:

• be displayed at all times (without any header),
• not use-up the sidebar (although an option to make the sidebar use the tab manager's space would be nice for people who prefer compactness),
• display a tab preview pop-up over the currently open page,
• properly support the drag-and-drop functionality for tabs
• and maybe even get style information of the currently selected browser theme (if that isn't already possible).

The browser would also have a "Tab Managers" setting, allowing the user to select one or more (!) tab managers to use from a list of installed ones, including the native ones. Separating the native horizotal and vertical tab managers in regard to that setting should also be considered.

[Jon]

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