This may not be the cause of your trouble, or it might only be part of the problem, but:
the tab function no longer works, merely showing the contents of the first tab on each other tab.
Is it possible that the tab control touches or crosses the boundary line between header and body layout parts?
Tab controls, slide controls, popovers and portals all have to "own" the objects that they contain. you can tell that this is the case because they will move with the containing object when that object is moved. I often nudge such an object a few pixels back and forth when working with the layout objects that it is supposed to "own", to be sure that they are still owned.
Certain actions such as moving an object far enough that part of it is outside the boundaries of the containing control can cause it to "fall" out of the container. Using the alignment tools in the inspector, however, can position objects so that they appear to be contained by such a control but the the control fails to "own" them. You have to move the object away from the control, release the mouse button and then drag it back, being careful not to let go of the mouse button until the object is fully inside the boundaries of the containing control. Once you've done that, you can "find tune" it's position within the control using the alignment tools.
Sorry not to have replied earlier. Have had a long absence from my desk. Thanks you for your helpful comments. I suspect you have put your finger on the problem. I am a relatively inexperienced Filemaker user - still learning the ropes. Andrew Hay