To put it another way, can you name any applications you feel do cross-platform right? Are there any example you feel tick all the boxes for you, and do they use any framework?
It's far from perfect, but Corel AfterShot Pro[0] is an example of commercial software that uses Qt and ships for Linux, Mac, and Windows.
Some of the imperfections I notice are:
* The options dialog has a lot of things that just look "different". The controls are native, but the way they're positioned, etc, clearly isn't. The Mac has a lot of common things, like which order the Yes/No/Cancel buttons would appear in a dialog box, that aren't done the same in AfterShot.
* Being a photo management/editing application, it naturally uses a dark theme that looks out of place everywhere. Adobe Lightroom and Apple's now-discontinued Aperture both do this. It seems to be a normal thing with this type of application.
* On Linux, the window decorations come from AfterShot, not from the window manager. It also has the Windows close/minimize/maximize buttons.
* On an old-enough Mac, the integrated GPU doesn't provide the OpenCL stuff that AfterShot requires. This causes it to crash at startup, whereas a real Mac application would tell the OS to fire up the discrete GPU.
* I seriously doubt that it exposes any of its API to AppleScript/COM/DBUS, but I haven't checked. I know that it doesn't register itself as a source for the system-wide photo browser on the Mac (as an example, Insert > Pictures > Photo Browser in Microsoft Word on the Mac will show your Aperture, Photos, and iPhoto libraries as whatever structure they have in that application, not just as files on disk).
Some of the imperfections I notice are:
* The options dialog has a lot of things that just look "different". The controls are native, but the way they're positioned, etc, clearly isn't. The Mac has a lot of common things, like which order the Yes/No/Cancel buttons would appear in a dialog box, that aren't done the same in AfterShot.
* Being a photo management/editing application, it naturally uses a dark theme that looks out of place everywhere. Adobe Lightroom and Apple's now-discontinued Aperture both do this. It seems to be a normal thing with this type of application.
* On Linux, the window decorations come from AfterShot, not from the window manager. It also has the Windows close/minimize/maximize buttons.
* On an old-enough Mac, the integrated GPU doesn't provide the OpenCL stuff that AfterShot requires. This causes it to crash at startup, whereas a real Mac application would tell the OS to fire up the discrete GPU.
* I seriously doubt that it exposes any of its API to AppleScript/COM/DBUS, but I haven't checked. I know that it doesn't register itself as a source for the system-wide photo browser on the Mac (as an example, Insert > Pictures > Photo Browser in Microsoft Word on the Mac will show your Aperture, Photos, and iPhoto libraries as whatever structure they have in that application, not just as files on disk).
[0] http://www.aftershotpro.com/en/products/aftershot-pro/