>>So far I found, that setting pageframe ActivePage to 1 solves the focus issue. However, it executes page activate and deactivate, which I'd like to avoid. Fabio's KEYBOARD suggestion produces undesired effect of empty comboboxes...
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>>I tried all other things: setting focus explicitly, playing with Form's GotFocus. Form's GotFocus works, but then somehow the first click on the different tab doesn't work, only the subsequential clicks.
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>Maybe because we use tabless pageframes. We control the .ActivePage through code, and the UIEnable works fine.

It's a different story with the tabless pageframe and especially if you're setting ActivePage through the code. Try to remove the first setting of the ActivePage and I'm sure you'll see the same effect as I do.