So, one thing that I am still disappointed about are the breaking changes made to GTK+ and SDL. Wow, well I wrote some software against these systems, and now it has been rendered obsolete. Where am I going to go next?
Luckily, I don’t need to look too far to answer this question. Windows API, Windows GDI, and X11, despite being feature limited and not feeling like your first choice when you want to write a new application rapidly, are still alive and well in modern operating systems, and they have maintained their backwards compatibility. And come on, these are more than 20 years old! Likewise, Qt is a rather old widget toolkit, but it has aged well with excellent backwards compatibility. So, the choice is clear. Don’t move forward to the new versions of GTK+ and SDL. Move sideways or even downward with Qt, Windows API, and X11.
Again, I reiterate, because this is important! Qt is one of the few to hold solid and true to backwards compatibility, and it has made them wildly popular in recent years when everyone else has failed.
Point in hand, if you are building a computer system from scratch, Windows API, X11, OpenGL, and Qt are the GUI and graphics standards to emulate. PostScript, PDF, and HTML rendering should be implemented as higher-level applications on top of these APIs. OpenCL and Vulkan… as I see it, those are for the bigger, more highly evolved systems.
How important is PostScript and PDF, you wonder? Very important. Unix old-timers loved printed manuals, and we can all see why before the advent of 4K monitors at reasonable prices. Early computer displays were standard definition resolution or lower, e.g. 640 x 480. With these resolutions, you cannot have fancy typography, which for many people can be quite an eyesore, not to mention that even with bare-bones typography, you still can’t see very much text simultaneously. If you want a workspace where you can see both the text of your documentation/manuals and your code at the same time, you will find yourself very short-handed.
UPDATE 2019-08-30:
So, I actually got around to reading the article on upgrading from SDL 1.2 to SDL 2, and I must say. The breaking changes are not as bad as I thought they were. But, nevertheless, compared to the other APIs I have mentioned, the fact that there have been breaking changes is what makes it different. So, overall, SDL is a pretty good and stable API, though not quite as stable as those other APIs.
20190830/http://wiki.libsdl.org/MigrationGuide
Likewise, another oversight that I was aware of, but did not mention when writing the original article was the breaking changes made between Qt 3 and Qt 4. Here is the corresponding migration guide for Qt. But, Qt 4 does provide a Qt 3 support library to ease porting to Qt 4. Also, there are some minor breaking changes between Qt 4 and Qt 5.
20190830/http://doc.qt.io/archives/qt-4.8/porting4-overview.html
20190830/https://wiki.qt.io/Transition_from_Qt_4.x_to_Qt5
20190830/https://en.wikipedia.org/wiki/Qt_version_history