This is a request for new `chrome.tabs.onXXXXX` API. My addon "Multiple Tab Handler" provides ability to select(highlight) tabs by dragging on tabs, and shows a popup menu when the mouse button is released. In other words, dragging on tabs is treated as area-selection for icons on the desktop. After XUL is ended, something alternative is required to migrate my addon to WebExtensions. And, the API should provide ability to cancel actions on tabs. It will allow addons to provide various custom actions, like Tab Mix Plus. In other words Tab Mix Plus will require this or similar API to be migrated.

Multiple Tab Handler: https://addons.mozilla.org/firefox/addon/multiple-tab-handler/