Tab Container Hacky Toggles — Ysaxa

This extension adds pinned tabs that act as buttons to show/hide all tabs pertaining to the relevant tab container (except the current active tab).
Since they are pinned tabs, when clicking them, the browser will actually switch to them. But then the extension will automatically switch back to the previously active tab, and show/hide relevant tabs. So you will probably see a flicker everytime you click a toggle-tab-button.

Does not work in private windows since it relies on containers.

Permissions are "tabs", "tabHide", "contextualIdentities" and "cookies" to be able to work with tabs and their containers (which are basically cookie store IDs).