How do you use the Ruler in the IE8 Developer Tools?

Question

I have no idea how the Ruler is supposed to work in the IE8 Developer Tools. I am obviously missing a simple step somewhere. Here's what I'm doing:

1. Open IE.
2. Load a web page.
3. Open the Developer Tools window.
4. In here, go to Tools -> Show Ruler
5. A little "Ruler" window appears in the top-left of the page in IE:

Now what? The cursor should turn into a cross and allow rulers to be drawn, but this isn't happening. Do I need to click somewhere special or press a magic key? (holding down Ctrl doesn't work)

Answer 1

OK, it seems to work if I double-click one of the four coloured squares (see image in the question), and then clicking Cancel the resulting colour-picker dialog. Clicking the OK button here does not result in a ruler. This can't be right can it?