That's the legacy code at action. Huge chunks of windows are ancient code bundled inside wrappers to keep it working, rather than refactoring the code itself. Regressions are stupidly common, partly as a result of that choice, partly because MS has haemorrhaged talent (and thus domain knowledge) since the early 2000s, from a combination of retirements and ever-growing bureaucracy and interdepartmental politics stifling creativity and innovation within the company.
They can't fix it now because the sort of people that MS used to have banging down their door all went to work for Google, then fucked off from there to start-ups, or moved onto other things while they wrote software as a hobby. They can't get the talent, and if they did manage to snag it, that talent would either end up beaten down or end up leaving again.