A Chrome extension to remove the menubar from the Slack webapp.

hide.css

.p-client.p-client--ia-top-nav {
  grid-template-rows: 0 auto min-content;
}

.p-client.p-client--ia-top-nav > .p-top_nav {
  opacity: 0.1;
}

.p-client.p-client--ia-top-nav > .p-top_nav:hover {
  opacity: 1;
  transition: opacity 0.2s;
}

manifest.json

{
  "manifest_version": 2,
  "name": "hide-slack-menubar",
  "description": "Hides Slack's new menu bar in the browser",
  "version": "1.0",
  "content_scripts": [
    {
      "matches": ["https://app.slack.com/*"],
      "css": ["hide.css"]
    }
  ]
}