Making the main scrollbar always visible

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;
}

html {
    overflow-y: scroll;
}

Is that what you want?

Unfortunately, Opera 9.64 seems to ignore that CSS declaration when applied to HTML or BODY, although it works for other block-level elements like DIV.

html {height: 101%;}

I use this cross browsers solution (note: I always use DOCTYPE declaration in 1st line, I don't know if it works in quirksmode, never tested it).

This will always show an ACTIVE vertical scroll bar in every page, vertical scrollbar will be scrollable only of few pixels.

When page contents is shorter than browser's visible area (view port) you will still see the vertical scrollbar active, and it will be scrollable only of few pixels.

In case you are obsessed with CSS validation (I'm obesessed only with HTML validation) by using this solution your CSS code would also validate for W3C because you are not using non standard CSS attributes like -moz-scrollbars-vertical

Setting height to 101% is my solution to the problem. You pages will no longer 'flick' when switching between ones that exceed the viewport height and ones that do not.

body { 
    min-height: 101vh; 
} 

works the best for me

I do this:

html {
    margin-left: calc(100vw - 100%);
    margin-right: 0;
}

Then I don't have to look at the ugly greyed out scrollbar when it's not needed.

html { height:initial!important; }

You may not need the !important - depends on what CSS is in place.