Now that the message header is fully HTML and the available customization is done in plain JS and CSS, we can experiment with some further actions in order to explore the ability to hide and show the available buttons.

A11Y aspect
The old XUL customization is not keyboard accessible so we need to offer the ability to customize those buttons via keyboard.

UX aspect

• Users might use the main toolbar with all action buttons in a vertical layout, therefore they don't need the same buttons in the message header.
• Users might never use some specific buttons and have limited screen space, therefore controlling what to show will improve their experience.
• Users might heavily rely on specific actions (tags, print, conversation, redirect), therefore having the ability to show those buttons outside the "more" dropdown will improve their efficiency.

Implementation overview

• In the Customize Popup, offer a menu item to enter a secondary panel.
• List all available buttons in this panel, with a checkbox to control their visibility.
• Add quick "Show all" and "Hide all" buttons to mass toggle.
• The "More" button shouldn't be removed.
• The order of those buttons shouldn't be changed (for now, we can explore this later).
• An array of "unchecked" (hidden) buttons should be saved in the XULStore.
• The array should contain only the buttons IDs, to be used as selectors to hide them.

This should be out of scope for 102 as it requires new strings and a bit more thinking