I’ve been working for months on a site whereby there is significant customization. Recently, I discovered that I cannot fully view the Modify Web Part form. Edit Page displays all the Zones and Web Parts properly. But as soon as the Modify Web Part option is selected, the returned page displays the body content in a short (~50px high) scrollable frame; and the modification form only has the header and buttons along the bottom. (The body content frame is equal in height to the modification frame height.) There are no scroll bars, so the content of the form is not visible at any time.
I have tried setting every table, row, and cell to a height of 100% which would make everything visible (like the Edit Page) if things were working properly. At this point, I have to guess that the problem is within a CSS setting. But, I have tried many settings without success. No doubt something got switched along the way that crippled this important SharePoint feature.
Add the following line to your CSS styling:
overflow: visible !important;