alt/meta-backspace is interpreted as esc + backspace

[lspitzner]

I have observed on multiple (but random) times that alt-backspace exits my program without any trace of an error message, instead of it doing what the program is supposed to do on alt-backspace (which is not to halt). Had me confused, but, looking at

specialSupportKeys =
    [ -- special support for ESC
      ("\ESC",EvKey KEsc []), ("\ESC\ESC",EvKey KEsc [MMeta])
    -- Special support for backspace
    , ("\DEL",EvKey KBS []), ("\ESC\DEL",EvKey KBS [MMeta])
    -- Special support for Enter
    , ("\ESC\^J",EvKey KEnter [MMeta]), ("\^J",EvKey KEnter [])
    -- explicit support for tab
    , ("\t", EvKey (KChar '\t') [])
    ]

This makes a good amount of sense - if the input buffer for some reason contains only \ESC (part of a \ESC\DEL) then the input sequence is interpreted as the sequence of \ESC (and \DEL once the next part is read). This depends on where the input is split up due to input-buffering internals, which of course is mildly random.

(A plain Esc is treated by my program to exit cleanly, which explains my initial observation.)

Does this diagnosis make sense so far?

[glguy]

In Terminal.app, The M-BS combination is correctly recognized as EvKey KBS [MMeta]. It will probably help if you specify which application is exhibiting this behavior.

[lspitzner]

@glguy Perhaps I was unclear. The observed behaviour is random. In like 95% of the input events everything is fine.

What is Terminal.app?

[lspitzner]

I just tested using the vty-demo executable contained within the package. When i keep alt-backspace pressed, the program exits randomly after, on vague average, 20 input events.

[glguy]

Terminal.app is the default terminal program on macOS. Using vty-demo in this program I get 100% success after a couple minutes of holding down M-BS. Knowing what environment you're testing this in will help with reproducing the problem.

[lspitzner]

I am running all this on archlinux.

Testing with different terminal emulators gives somewhat differing results:

• xterm(330)
  detects alt-backspace as EvKey (KChar \136) [] and consequently does not seem to fail at all
• xfce4-terminal-0.8.6
  ~5% probability to fail
• LXTerminal-0.3.0
  ~3% probability to fail
• GNOME Terminal 3.26.1 Using VTE version 0.50.1 +GNUTLS
  ~1% probability to fail (sometimes runs for more than a minute, but i have observed 2 exits)
• a non-emulator terminal (what software runs that? is it just the kernel? i don't even know..)
  could not reproduce

[lspitzner]

Oh, and I also see EvKey KBS [MMeta] as the "correct" output. I don't mind the alt-vs-meta swap.

[lspitzner]

I am rather certain that if I modified vty-demo to not exit on <esc> i would be able to observe the separate <esc> and <backspace> events. I have done that test in a different program, but not using a modified vty-demo.

[milesrout]

The encoding of <esc> then <backspace> is the same as the encoding of <alt-backspace>. Alt/Meta is encoded as a prefixed . I believe programs usually end up detecting whether it's <Alt-x> vs <Esc>x using a delay: if the ESC and the next key are far enough apart, they're considered separate, otherwise it's an <Alt-x>.