How to always show the vertical scrollbar in a browser?

jQuery shouldn't be required. You could try adding the CSS:

body    {overflow-y:scroll;}

This works across the latest browsers, even IE6.

Just a note: In OS X Lion, overflow set to "scroll" behaves more like auto in that scrollbars will only show when being used. They will disappear when not in use. 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.