When I tried to do the same with an ordinary panel - The designer
didn't let me change properties like size, location, etc. in the
inherited form but I could add controls to that panel. The designer
did "see" the panel and focus it (with a read only kind of frame).
It is not the same with my tab control - I can see it, but the
designer ignores it completely. When I click it the form itself gets
the focus - like the tab control is not there.

Hi Roee,

try this: Make the control protected on the form that you inherit from.

I suppose, the designer follows the standard inheritance pattern:
When you define a derived object you may change the sub-objects that are
declared as protected or public and may not change those declared as private.
And in does not metter how you define a new derived object (a new form):
(i) you edit the VB/c# text; or
(ii) you do this with the help of a designer.

If a sub-object is private, a designer may not see it at all.
After all, a control is nothing more but a property of the form it is
placed on (a special one, but still a property).
You may see a private sub-object "phisically" (it is drawn), but no one
may access/override this "property" but its original owner.

I suppose also that it does not metter is a control made protected or
public (i mean, the designer of a derived form can edit it in both cases).
Declaring it public will have other consequences (as for any other property)
but does not metter w.r.t. designers of an inherited form.

HTH,
Dima.