The manual isn't quite correct - the minimum parallax theme image height is 15% of the viewport width. Because of the weird CSS that's going on, including CSS perspective and 3-d transforms, there's really no way to say exactly how high the image will be. It will be different on different monitors. On top of that, it's a background image that's set to cover the space. Very messy - it's a bit of a miracle that it works at all.


I think it's just that I'm using a fixed value in the script that handles the TofP icon, but the page layout is highly variable. I'll need to tinker a bit, though without getting overly complicated, it won't be able to handle a mix, i.e., a big parallax theme image on some pages, but no theme (i.e., just a colored banner) on other pages. I'd rather not start passing variables into the script on every page. But the intent is not to have it show up at a precise moment - it's more of an "it's there roughly when it's convenient to get back to the title bar navigation links" thing.


I wondered about the proper "low end" value for the parallax theme image when I created Pluto (the parallax routine in Mars and Venus is pretty much just copied from Pluto). I settled on letting it go as far down as it still worked (below 15%, bad things start happening), but as a practical matter, it should probably bottom out at something like 30%. Less than that, and it may not be worth the effort - the effect tends to get lost.

The next releases of most of my skins will include a revised threshold for the appearance of the "top of page" icon when using either a fixed-height theme image, or no theme image (the threshold was OK for a parallax theme image). It was showing up a little too early.