Windows has keyboard shortcuts on buttons, but you can set per-user whether they appear underlined always or only when you hold Alt. Mac always has exactly one, immovable menu bar; and Enter doesn't close a dialog box. GNOME can rollup or pin windows from the title bar if you add the buttons for it. Whether the native dialog layout has the ok button to the left of the cancel button depends on the OS and the writing direction (p206 of [1]). Not every platform has a native calendar widget. And so on ...
Worse still, you're building an abstraction layer on top of several genuinely different systems, that users of different platforms will expect to work differently in the first place. Go down this rabbit hole and you end up with "show a help button on modal dialogs only on platforms where this is usual, and where you can open the help window without closing the modal first". "Portable native-GUI" is almost an oxymoron; wxWidgets is perhaps the closest we can get where at least you can ask for a wxFindReplaceDialog abstraction (p229 of [1]) and get the cancel button in the platform-default place, even if "Find-Replace Dialog" is not a feature included in the OS' component library itself.
[1] https://wxwidgets.org/docs/book/Cross-Platform%20GUI%20Progr...