Retina display support for XQuartz X11 applications

Yes. X11 is very much still in use as it is a very powerful windowing system and there is nothing that Apple has released that replaces the core functionality of X11.

I wouldn't spend time learning X11 in the abstract until you have a need to produce code that runs on X11 or you need to run software that is written for X11.

In a nutshell - it is a very flexible client/server system. The server runs where the keyboard and mouse and display are. The client is the program running locally or remotely on a different computer. For example I can run protein folding engines on large servers that are in a university across the country (or world) and interact with the user interface in a cafe on my MacBook Pro.

I have a choice of a wide variety of window managers to use. The virtual and extended desktop concepts in X11 are very different than Mac's typical you only get a screen if something is physically connected or enable spaces.

There are several downsides in that the system is different and to use it, you have to do some learning. Also - it doesn't map perfectly to Apple - cut and paste can be disorienting, mouse buttons have different functions, keyboard mapping can be tricky, and the uniformity of the Mac interface is disturbed as most X11 programs come with a wide variety of widgets. Even the most die hard fan of X11 chrome has to admit that a lot of it is "quaint" and some of it is really hard to use for people that expect the Macintosh user interface experience. (This is more of a culture thing - some countries have delicacies that are off-putting to others - it's not right or wrong - just different heritages and long assumptions on how something is "supposed to be done".)

Cheers and good luck!

I installed the XQuartz application. It was in /Application/Utilities/XQuartz.app. I just renamed it from XQuartz.app to X11.app and everything works fine, so far.

Update: don't install XQuartz for use with No Machine. I opened a text editor and started typing, all the keys were screwy. For instance, the Delete key was now comma, all the letters were transposed, general chaos. I replaced XQuartz with the official X11User.dmg package and this keyboard problem went away. One more step I had to take: when on the remote desktop I tried to cd into a different directory, and hitting the "d" key kept showing me the desktop. If you want to cd, you will need to go into System->Preferences->Keyboard shortcuts. Scroll down, and notice an entry "Hide all windows and show desktop" which is mapped to "D". Change this to Alt+D or Ctrl+D and cd away!