It has pleased western historians to write of a decline of the Ottoman Empire as though a stable and uncontested polity of that name once existed. The borders did expand and contract but they were always dynamic and always in "question" right from the beginning. The Ottoman Empire was created from the lands of the former eastern Roman Empire on the occasion of the latter's violent demise. The last Roman emperor died fighting hand-to-hand in the streets of his capital, Constantinople, overwhelmed by the Ottoman military, in May, 1453. The victors inherited his remaining territory in the Balkans.
What was always in question from the beginning of the Ottoman Empire? 
The borders