manifest.json

{

  "manifest_version": 2,
  "name": "Slack hide panes",
  "version": "1.0",
 
  "description": "Hide Slack Panes.",

  "applications": {
    "gecko": {
      "id": "slack-hide-pane@javascript.ru",
      "strict_min_version": "45.0"
    }
  },

  "content_scripts": [
    {
      "matches": ["*://*.slack.com/*"],
      "js": ["slack-hide-panes.js"],
      "css": ["slack-hide-panes.css"]
    }
  ]

}

slack-hide-panes.css

/* hide top */
.slack-hide-panes #client_body {
  margin-top: 0;
}

.slack-hide-panes #client_header {
  display: none;
}

/* hide left */
.slack-hide-panes #col_channels_bg,
.slack-hide-panes #col_channels {
  display: none;
}

.slack-hide-panes #messages_container {
  margin-left: 0;
}
.slack-hide-panes #footer {
  left: 0;
}

slack-hide-panes.js

document.addEventListener('keydown', function(e) {
  if (e.keyCode == 115) {
    // F4
    toggle();
  }
})

function toggle() {
  document.documentElement.classList.toggle('slack-hide-panes');
}

window.addEventListener('resize', function() {
  if (!document.documentElement.classList.contains('slack-hide-panes')) return;

  setTimeout(function() {

    var scroller = document.getElementById('msgs_scroller_div');
    console.log(scroller.offsetHeight);
    scroller.style.height = scroller.offsetHeight + 50 + 'px';
    console.log(scroller.offsetHeight);
  }, 100);

});