Making the main scrollbar always visible

Things have changed in the last years. The answers above are not valid in all cases any more. Apple is pushing disappearing scrollbars everywhere. Safari, Chrome and even Firefox on MacOs (and iOs) only show scrollbars when actually scrolling — I don't know about current Windows/IE. However there are non-standard ways to style scroll bars on Webkit (IE dropped that a long time ago).

Make sure overflow is set to "scroll" not "auto." With that said, in OS X Lion, overflow set to "scroll" behaves more like auto in that scrollbars will still only show when being used. So if any the solutions above don't appear to be working that might be why.

This is what you'll need to fix it:

::-webkit-scrollbar {
  -webkit-appearance: none;
  width: 7px;
}
::-webkit-scrollbar-thumb {
  border-radius: 4px;
  background-color: rgba(0, 0, 0, .5);
  -webkit-box-shadow: 0 0 1px rgba(255, 255, 255, .5);
}

You can style it accordingly if you don't like the default.

html {
    overflow: -moz-scrollbars-vertical; 
    overflow-y: scroll;
}

This makes the scrollbar always visible and only active when needed.

Update: If the above does not work then just using this may.

html {
    overflow-y:scroll;
}