Arduino Keyboard - print() works but println() does not
Please test: Keyboard.print("Hello world.\n");


...


  That works. println still doesn't.


I'm going to guess here that println is doing something odd in the Yun libraries. I note that the Keyboard.cpp file in my release has this in it:

#define SHIFT 0x80
const uint8_t _asciimap[128] =
{
    0x00,             // NUL
    0x00,             // SOH
    0x00,             // STX
    0x00,             // ETX
    0x00,             // EOT
    0x00,             // ENQ
    0x00,             // ACK  
    0x00,             // BEL
    0x2a,           // BS   Backspace
    0x2b,           // TAB  Tab
    0x28,           // LF   Enter
    0x00,             // VT 
    0x00,             // FF 
    0x00,             // CR    &lt;---------- note!


Notice that linefeed has a code mapped (0x28) but carriage return (CR) does not (unless you call 0x00 a mapping).

I think you'll find that when you hit Enter on a keyboard it is sending a CR (after all, it used to be labelled the "return" key) and not a LF.

However I thought that the Print class outputted both:

size_t Print::println(void)
{
  return write("\r\n");
}


Maybe yours doesn't. I'm not certain which exact println will be pulled in for a Yun. The fact that it used to work, and now doesn't, suggests a change there somewhere.