Firstly, your VST situation will be that the plugin, the host or both are not implementing things correctly: one or the other has claimed DPI-awareness inappropriately. That’s on them.
Secondly, you misunderstand the relative situations of macOS and Windows. The difference is not to do with their approach to scaling, but rather their approach to supporting old software, and the diversity of hardware. Social factors, not technical. Apple could absolutely have done fractional scaling, and it’d have worked just fine for them; the present situation in how well high-DPI stuff works across platforms would be indistinguishable: macOS would still have basically nothing broken, and Windows would still have quite a few widely-used pieces of software getting it wrong. Here’s why:
Apple developers are very probably using a high-DPI screen, so they’ll immediately notice if anything is off. Apple is controlling, and anything published through their store will have any obviously-bad scaling noticed and rejected. Apple doesn’t care about long-term compatibility, so earlier software that didn’t implement scaling properly largely just doesn’t run at all any more.
Windows developers are probably using a low-DPI screen, and are publishing to a largely unmoderated platform that still has software from decades ago run just fine.
Functionally there’s no difference between fractional and integral scaling in how well software will support them. If you try to implement things completely from scratch, fractional will be a little harder because you have more rounding decisions, but in practice you always use a library that takes care of that for you (normally a full widget toolkit, but VST plugins will often use something somewhat lower-level but still above the physical-pixels layer except when you deliberately opt for fine control), so there is literally no difference at all. The difference in how well things work on these different platforms is purely other factors.
Firstly, your VST situation will be that the plugin, the host or both are not implementing things correctly: one or the other has claimed DPI-awareness inappropriately. That’s on them.
Secondly, you misunderstand the relative situations of macOS and Windows. The difference is not to do with their approach to scaling, but rather their approach to supporting old software, and the diversity of hardware. Social factors, not technical. Apple could absolutely have done fractional scaling, and it’d have worked just fine for them; the present situation in how well high-DPI stuff works across platforms would be indistinguishable: macOS would still have basically nothing broken, and Windows would still have quite a few widely-used pieces of software getting it wrong. Here’s why:
Apple developers are very probably using a high-DPI screen, so they’ll immediately notice if anything is off. Apple is controlling, and anything published through their store will have any obviously-bad scaling noticed and rejected. Apple doesn’t care about long-term compatibility, so earlier software that didn’t implement scaling properly largely just doesn’t run at all any more.
Windows developers are probably using a low-DPI screen, and are publishing to a largely unmoderated platform that still has software from decades ago run just fine.
Functionally there’s no difference between fractional and integral scaling in how well software will support them. If you try to implement things completely from scratch, fractional will be a little harder because you have more rounding decisions, but in practice you always use a library that takes care of that for you (normally a full widget toolkit, but VST plugins will often use something somewhat lower-level but still above the physical-pixels layer except when you deliberately opt for fine control), so there is literally no difference at all. The difference in how well things work on these different platforms is purely other factors.