[Bug 1562308] [NEW] missing or duplicate lines caused by a wrapped line with wide characters

[Launchpad Bug Tracker]

You have been subscribed to a public bug by Mitsuya Shibata (cosmos-door):

When you scroll down text with "j" key and encounter a wrapped line
with wide characters (such as UTF8-encoded Japanese characters),
"less" seems to show the whole wrapped line at a single "j" key,
causing the view scroll down by 2 lines at once. Strangely, if you
type "k" to scroll up, it does that by only 1 line. As a result, there
is a missing line that should have been shown.

Even stranger stuff (i.e., duplicate lines) happens when you type "j"
and "k" alternately when a wrapped line with wide characters is at the
bottom of the view.


# Steps to reproduce

1. Open xterm.
2. Set the geometory of xterm to 71x22.
3. Open the attached lesstest.long_jap.txt with less (maybe you need
   environment variable LANG=ja_JP.UTF-8)
4. Type "j", then you will see "003" at the top, and a long wrapped
   line with Japanese characters at the bottom.
5. Type "k", then you will see "001" and "003" at the top. "002" is
   missing.

# Expected behavior

In step 4, only the first part of the wrapped line should be shown.

In step 5, all "001", "002" and "003" should be shown.


# Test Environment

- Xubuntu 16.04 (Xenial) beta (in VirtualBox on Xubuntu 14.04)
- less: 481-2.1
- xterm: 322-1ubuntu1
- Japanese environment (LANG=ja_JP.UTF-8)


# Note

- The same problem happens on xfce4-terminal (0.6.3-2ubuntu1)
- lv (4.51-2.3build1) doesn't have such problem.
- In "less" in Xubuntu 14.04 (version 458-2), this problem didn't
  exist.

** Affects: less (Ubuntu)
     Importance: Undecided
         Status: In Progress

-- 
missing or duplicate lines caused by a wrapped line with wide characters
https://bugs.launchpad.net/bugs/1562308
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