Heh, it's been mixed reactions. I will probably tweak the initial experience, but will need some time because I can't leave it open on load because it blocks the header.

I thought it was nifty. The shocking aspect might just be the big layout change. Perhaps a notification ("Check out the demo") so the user has to click it to enable the side pane, and then its business as usual.

I liked the layout as well. One thing I would change would be to make the code area wider than the demo area, as some of your lines wrap, where the demo area rarely needs a line break.

It's a pretty crappy experience. I'm reading an article, suddenly the text decides to move left for 'no good reason' to create a narrow but empty sidebar.

WTF? I don't like reading things on the left side of my monitor, so I make the window wider to try and put the text back in the middle. Except now the demo area gets even wider (why? you don't use it for anything but tiny things?).

As I scroll, the various iframe loads create annoying jank, and to top it off, there's a bug at the end of the post where it moves horizontally for no good reason.

If you're going to do stuff like this, you need to polish it waaay better. This reminds me of the UI animations in the new Firefox. A good idea ruined by implementation, would've been better to leave them out.

That demo area sliding out actually caused me to actually read the article. I was just about to close it with "meh, react"

Sometimes a shock is good. I took it as, "Hey, this page just gained new abilities." Then I clicked and moused over the square and the triangle to see what they were about.