A web viewer will always be in the front of everything else on the layout (except for an active field - but you cannot activate a field by clicking "through" the web viewer, unless it's empty).
Not sure what you're trying to accomplish here - perhaps a portal could do?
Here's some more info: On a layout "snakes" in "view as form" mode, I would like to add a picture of the snake. (Each snake has a record in the "sakes" table that is shown on that layout). Since these images are all very wide, much wider than I would like the overall layout to be, the image should be scrollable in horizontal direction. I do not want to rescale the image to fit the layout. Instead, it's supposed to be shown at full resolution. This is why I'm currently doing it in a web viewer. Web viewers allow to show the image at full resolution in a smaller area, with scroll bars. Normal image fields don't, you can only choose between rescaling to fit and cutting.
On top of that image, I would like to draw horizontal lines with some text indicating how fat the snake is. Like a ruler. These lines and labels are supposed to stay where they are upon scrolling, so that the labels aren't scrolled out of sight.
I can't see how portals should do what I want.
Let me know if I'm still being somewhat unclear, I can explain further.
Well, you can't overlay the web viewer with grid lines. However, you could probably do this inside the web viewer with CSS.
Hm! That's an interesting idea.