emacskeyboardmacosshortcut
I can no longer use Command + Space inside iTerm2.
I have been using iTerm2 for years, and configured it so that the Command keys acts as the Alt key. I recently applied the security patch for macOS Sierra 10.12.6, and while all other key combinations continue to work, Command + Space is no longer transmitted to the terminal application, which is a pain as it is bound to set-mark in my favorite environment, qemacs.
Where can I change this behavior?
See the options under M-x customize-group ns
There you can adjust the behavior of the modifier keys. I have set the right alt to "No modifier" - that's the same behavior as on PC keyboards with Alt-Gr.
Another setting I like is to unset both alt keys and use the function key instead for alt.
Almost all of the keyboard shortcuts you've learned on Windows are the same on the Mac. The difference is which modifier key you use to invoke them.
On Windows, shortcuts are typically invoked with the Ctrl modifier key.
On the Mac, you use the 
key, also known as the "command" key. It's in the same physical position as the Alt key on a Windows keyboard, but is nothing like the Alt key.
The Mac has its own Alt key, labeled Opt/Alt. Its primary use is a secondary modifier for keystrokes. Command-Option-S might be "Save As", for example, while Command-S is regular "Save".
The Ctrl key is there on a Mac keyboard, too, but it's hardly ever used for anything.
The relative layouts look something like this:
PC: Ctrl Windows Alt
Mac: Ctrl Opt/Alt
The problem is that you're most likely using a PC keyboard on your Mac. In that specific case, the Opt/Alt and keys are reversed. You can change this in System Preferences, under Keyboard. Truly a huge pain in the rear for switchers. That layout looks like this, instead:
PC keyboard on Mac: Ctrl Opt/Alt
Apple's website has some pretty good hints and tips for Windows switchers. Start here for answers to your specific question about keyboard shortcuts. Then go here and read everything!
Best Answer
The system has defined a shortcut that intercepted this on my mac:
This setting might have been in effect already, but since I have only a single input source (US QWERTY keyboard), it may have been ignored before.
In any case, deselecting the shortcut fixed the problem. Since it can be a pain to have to go fishing for a binding in all panels, here is an app (recommended by bmike) that shows you the current live shortcuts in any app as it runs.