I am having a similar issue. The menu is vertical and on the left side of the page, and the submenu opens to the side of the menu. My client wants it to open inside the menu, pushing the other parts of the menu down. This is what they want, as an example:
Submenu closed:
Page A
Page B
Page C
Submenu open:
Page A
subpage a
subpage b
Page B
Page C
I assume my problem is happening for the same reason as the others here, though it is going to upset my client. I hope they understand.