How do I get the real .height() of a overflow: hidden or overflow: scroll div?

Another possibility would be to place the html in a non overflow:hidden element placed 'out' of screen, like a position absolute top and left less than 5000px, then read the element's height. It's ugly, but works well.

Use the .scrollHeight property of the DOM node: $('#your_div')[0].scrollHeight

For more information about .scrollHeight property refer to the docs:

The Element.scrollHeight read-only attribute is a measurement of the height of an element's content, including content not visible on the screen due to overflow. The scrollHeight value is equal to the minimum clientHeight the element would require in order to fit all the content in the viewpoint without using a vertical scrollbar. It includes the element padding but not its margin.