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

[Łukasz Zemczak]

** Also affects: less (Ubuntu Yakkety)
   Importance: Undecided
       Status: New

** Also affects: less (Ubuntu Xenial)
   Importance: Undecided
       Status: New

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Title:
  missing or duplicate lines caused by a wrapped line with wide
  characters

Status in less package in Ubuntu:
  Confirmed
Status in less source package in Xenial:
  New
Status in less source package in Yakkety:
  New

Bug description:
  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.

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