1352692 - [meta] Update navigation toolbar overflow panel for Photon

Status: NEW

(Firefox :: Toolbars and Customization, enhancement)

Description

[:Gijs (out for now; he/him)]

Attachment: Mock of new overflow panel

Per the specs:

- it should be possible to move items in/out of the overflow panel permanently
- its button should be visible iff there are items (permanent or 'dynamically overflowed' ones) in the overflow panel
- the dynamically added and permanent buttons should be separated by a separator (permanent ones at the bottom)
- the panel should be visible and customizable in customize mode
-- in customize mode, dynamically added items (because the window is small) don't show up and can't be reordered. The user would have to make the window wider and then reorder them on the toolbar (this is the status quo).
- the panel should have a footer that opens customize mode
- the panel should probably use a panelmultiview to allow buttons that have subviews to work the way they currently do in the main ("hamburger") panel.


Open questions:
- what happens when there are "too many" items in this panel (for it to fit on the screen, or > some limit) ? Should the panel scroll (except for the footer, of course), or something else?