core: do not display non printable chars, fix function utf8_char_size_screen

Now the function utf8_char_size_screen can return -1 when the char is not
printable.

It has a specific behavior for some chars:

- U+0009: value of option weechat.look.tab_width
- U+0001 to U+001F (except U+0009): 1
- U+00AD (soft hyphen): -1
- U+200B (zero width space): -1

1 files changed, 18 insertions, 4 deletions

diff --git a/doc/en/weechat_plugin_api.en.adoc b/doc/en/weechat_plugin_api.en.adoc
index 3a2c5cba3..ecd867599 100644
--- a/doc/en/weechat_plugin_api.en.adoc
+++ b/doc/en/weechat_plugin_api.en.adoc
@@ -894,11 +894,12 @@ This function is not available in scripting API.
 
 ==== strlen_screen
 
-_WeeChat ≥ 0.4.2._
+_WeeChat ≥ 0.4.2, updated in 3.8._
 
 Return number of chars needed on screen to display UTF-8 string.
-Non-printable chars have a width of 1 (this is the difference with the function
-<<_utf8_strlen_screen,utf8_strlen_screen>>).
+
+WeeChat color codes are skipped and don't count in the result (this is the only
+difference with the function <<_utf8_strlen_screen,utf8_strlen_screen>>).
 
 Prototype:
 
@@ -3639,6 +3640,8 @@ This function is not available in scripting API.
 
 ==== utf8_char_size_screen
 
+_Updated in 3.8._
+
 Return number of chars needed on screen to display UTF-8 char.
 
 Prototype:
@@ -3654,7 +3657,18 @@ Arguments:
 
 Return value:
 
-* number of chars needed on screen to display UTF-8 char
+* number of chars needed on screen to display UTF-8 char:
+** -1: non printable char
+** ≥ 0: printable char
+
+The result is the return value of function `wcwidth` (see `man wcwidth`), with
+exception for the following chars, that have a specific behavior in WeeChat:
+
+* U+0009 (Tabulation): value of option
+  link:weechat_user.en.html#option_weechat.look.tab_width[weechat.look.tab_width ^↗^,window=_blank]
+* U+0001 (1) to U+001F (31), except U+0009 (Tabulation): 1
+* U+00AD (173, soft hyphen): -1
+* U+200B (8203, zero width space): -1
 
 C example: