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A user sent a transcript from a curses-based tool 'ncmpc', which carefully disables terminal autowrap when printing a character in the bottom right corner of the display, and then turns it back on again. After that, it expects that sending the backspace character really moves the cursor back a space, instead of clearing the wrapnext flag. But in PuTTY, we set the wrapnext flag even if we're not in wrapping mode - it just doesn't _do_ anything when the next character is sent. But it remains set, and still affects backspace. So the display is corrupted by this change of expectation. (Specifically, ncmpc is printing a time display [m:ss] in the very bottom right, so it disables wrap in order to print the final ']'. Then the next thing it needs to do is to update the low-order digit of the seconds field, so it sends \b as the simplest way to get to that character. The effect on the display is that the updated seconds digit appears where the ] was, instead of overwriting the old seconds digit.) This is a tradeoff in desirable behaviours. The point of having a backspace operation cancel the wrapnext flag and not actually move the cursor is to preserve the invariant that sending 'x', backspace, 'y' causes the y to overprint the x, even if that happens near the end of the terminal's line length. In non-wrapping mode that invariant was bound to break _eventually_, but with this change, it breaks one character earlier than before. However, I think that's less bad than breaking the expectations of curses-based full-screen applications, especially since the _main_ need for that invariant arises from naïve applications that don't want to have to think about the terminal width at all - and those applications generally run in _wrapping_ mode, where it's possible to continue the invariant across multiple lines in any case.
PuTTY source code README
========================
This is the README for the source code of PuTTY, a free Windows and
Unix Telnet and SSH client.
PuTTY is built using CMake <https://cmake.org/>. To compile in the
simplest way (on any of Linux, Windows or Mac), the general method is
to run these commands in the source directory:
cmake .
cmake --build .
These commands will expect to find a usable compile toolchain on your
path. So if you're building on Windows with MSVC, you'll need to make
sure that the MSVC compiler (cl.exe) is on your path, by running one
of the 'vcvars32.bat' setup scripts provided with the tools. Then the
cmake commands above should work.
To install in the simplest way on Linux or Mac:
cmake --build . --target install
On Unix, pterm would like to be setuid or setgid, as appropriate, to
permit it to write records of user logins to /var/run/utmp and
/var/log/wtmp. (Of course it will not use this privilege for
anything else, and in particular it will drop all privileges before
starting up complex subsystems like GTK.) The cmake install step
doesn't attempt to add these privileges, so if you want user login
recording to work, you should manually ch{own,grp} and chmod the
pterm binary yourself after installation. If you don't do this,
pterm will still work, but not update the user login databases.
Documentation (in various formats including Windows Help and Unix
`man' pages) is built from the Halibut (`.but') files in the `doc'
subdirectory. If you aren't using one of our source snapshots,
you'll need to do this yourself. Halibut can be found at
<https://www.chiark.greenend.org.uk/~sgtatham/halibut/>.
The PuTTY home web site is
https://www.chiark.greenend.org.uk/~sgtatham/putty/
If you want to send bug reports or feature requests, please read the
Feedback section of the web site before doing so. Sending one-line
reports saying `it doesn't work' will waste your time as much as
ours.
See the file LICENCE for the licence conditions.