This stuff is just goofy. The inventory window has some odd dimensions that I just can't seem to figure out. The inventory page is 388x401. I found the source of the page by looking at the WEIGHT label which is at 0x379, and the name label, which is at 4,2. As you can see in the image below, the buttons go outside of that bounding box, but yet increases the page height to match their height + 1 spare pixel. The top left corner of the skills button is 1, 402, outside of the pages bounding box.
I think I'm missing some key behavior of the Page/TabBox elements that are important for it's size.