I've got my monitor in portrait mode and when I'm editing a question, I seem to gravitate towards the preview and start trying to click around to edit it.
That's a Newbie Mistake
I'm very surprised - I'm a developer building his own application, I love computers and I'm a power user who can recite most of the keyboard shortcuts for the apps I love. And I've been using Stack Overflow for quite a while now. You'd have thought I got used to it.
This the kind of thing I'd expect a user to do who doesn't understand about how browsers or editors work. And yet I've made this mistake three times in a row now and it's getting really annoying.
What Causes This
I think it's because:
The preview is large relative to the question editing area in portrait mode.
The distracting nature of the code preview or images - they stand out and draw my eye to the preview section.
I think if point 1 was fixed, point 2 would be a non-issue - because more of your preview would be pushed off the screen.
This could be done only in portrait mode - people viewing the page on a normal landscape monitor would be unaffected.