Drupal 7 dashboard doesn't fit on screen when using custom themes

I'm using Drupal 7 for a site I'm making, but when I am using the dashboard/backend of drupal the modal box doesn't fit on my screen and causes a wierd scrollbar (FF & Chrome etc) bug that I've never seen before on any site!

See how the bottom down arrow has disappeared?

On some screens this means that I can't see the bottom of the page, hiding the save buttons.

It works fine with drupal standard themes, bu开发者_开发知识库t not on my Zen subtheme.

Has anyone seen this before? or has anyone got a theory on how to fix this?

Thanks in advance.

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You can get rid of the overlay completely by disabling the Overlay module.