3.1.x: Mangled input/output when calling non-cygwin programs

[m0viefreak <m0viefreak.cm@googlemail.com>]

Hi,

Since Cygwin 3.1's pseudo console support was introduced I've run into
lots of issues with non-cygwin programs.

1) Mangled output: See screenshot [1].
   As you can see, at what it seems random places, the output is broken.
   The above can easily be reproduced using Apache Maven in any project,
   e.g. using a sample project created using [2].

2) Terminal width: For some reason, non-native programs do not use the
   full width of the terminal (Mintty and TERM=xterm-256color). Can also
   be seen in [1] (blue highlight) and reproduced e.g. using Maven with
   a relatively small window.

3) Typed characters while non-cygwin program is running are lost.
   Usually, when typing characters while a native program executes,
   which does *not* read from standard input, after the program exits,
   all these typed characters are put into the prompt.
   When executing a non-native program, such as, again, "mvn clean",
   which does not read from stdin, typed characters are completely lost.
   This is very frustrating, since I am often quicker at tying the
   next command, while the previous command is still doing something.

4) Importance of TERM: When connecting to my Cygwin installation using
   SSH using Putty, TERM=putty-256color is set. When executing non-
   native program from that session using that TERM, output is broken
   even worse than in 1), and also keybindings are broken. I need to
   force TERM=xterm-256color for those invocations.
   Shouldn't this be handled transparently and enforced automatically by
   the pcon code?

Note 1: This is tested using the latest Snapshot 2020-01-31 18:10:26 UTC
        from https://cygwin.com/snapshots/.
Note 2: It seems that the problems can be solved by setting
        CYGWIN=disable_pcon but it feels like there is some bug in the
        PTY code (not in the non-native program being called).

[1] https://images2.imgbox.com/dc/2d/MQwSH9S4_o.png
[2]
http://maven.apache.org/guides/getting-started/index.html#How_do_I_make_my_first_Maven_project

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