Screenshot showing excess white space between captcha and refresh link
Per request - I've selected the captcha so that you can see where it ends and gap between it and the refresh link.
The CAPTCHAs vary in height. I put a min-height of 95 (which seems to be a close upper bound) to accomplish two goals:
If this seems to be worse than jumping, we could look at removing the min-height. Or, we could change the CAPTCHA generation script to output fixed height images (a bigger project).
Fix is backported to wmf4. The captcha is a little closer because it's inside the fancycaptcha-image-container, whose min-height we set to 95 so the rest of the form doesn't (well, rarely) shift up and down each time you refresh the captcha image.