In the nineties when the antitrust case was building against Microsoft, various accusations of abuse of monopoly power were leveled against the company, some of which were proven true.

One of the more persistent accusations was that Windows contained some useful APIs that Microsoft kept secret, for use only by its own applications, to give them an unfair advantage against rival applications. So this presumably implies that e.g. Microsoft Word was better than WordPerfect because Word was calling some undocumented Windows API functions that the WordPerfect developers didn't know about.

This always struck me as rather odd. What, exactly, would those secret APIs do, that you couldn't do with the documented APIs? And the Windows division was trying hard to make their operating system attractive to third-party developers. (This was not just pro forma. They were really trying, with much success. "Developers, developers, developers!") Would they really be willing to degrade the quality of their operating system, for the sake of their buddies over in the applications division? That would require a level of interdepartmental cooperation that would be at least mildly unusual in a company the size of Microsoft.

I don't remember the claim ever being definitely resolved either way, just fading away as the industry moved on to focus on other things. But I might have missed something.

Did Microsoft really reserve any such secret APIs for its own applications? Or – it's hard to prove a negative across the board, but were any instances of the claim ever resolved in the negative down to a level of detail like 'oh yes, Word did poke such and such implementation detail of Windows, but this was invalid, and the Windows team was just as miffed when they found out about it as they would've been about a third-party developer doing it, here's an internal email transcript to prove it'?