When the height of the RadEditor is too small to accommodate the tools with the given width calling repaint() causes the appearance of the control to break due to incorrect calculations. This means that the control has an optimal (minimal) height.
The repaint() method is automatically called when the control is shown after being initially hidden on the page (e.g. after a partial postback)

Possible workarounds:
- increase the height of the control. For example, 640px width with 264px height and the default set of tools work well
- reduce the number of used tools. Effectively leaves more height for the control to use
- use ToolbarMode=ShowOnFocus to avoid having the tools in the main content initially, they will be shown in a popup when the content area is focused