Add support for fish_ambiguous_width

fish_ambiguous_width is a variable which controls the width of ambiguous CJK
characters.

Fixes #5149

doc_src/index.hdr.in

@@ -858,6 +858,8 @@ The user can change the settings of `fish` by changing the values of certain var
 
 - `fish_emoji_width` controls the computed width of certain characters, in particular emoji, whose rendered width varies across terminal emulators. This should be set to 1 if your terminal emulator renders emoji single-width, or 2 if double-width. Set this only if you see graphical glitching when printing emoji.
 
+- `fish_ambiguous_width` controls the computed width of ambiguous East Asian characters. This should be set to 1 if your terminal emulator renders these characters as single-width (typical), or 2 if double-width.
+
 - `fish_escape_delay_ms` overrides the default timeout of 300ms (default key bindings) or 10ms (vi key bindings) after seeing an escape character before giving up on matching a key binding. See the documentation for the <a href='bind.html#special-case-escape'>bind</a> builtin command. This delay facilitates using escape as a meta key.
 
 - `fish_greeting`, the greeting message printed on startup.

src/env.cpp

@@ -788,6 +788,16 @@ static void handle_change_emoji_width(const wcstring &op, const wcstring &var_na
     g_fish_emoji_width = std::max(0, new_width);
 }
 
+static void handle_change_ambiguous_width(const wcstring &op, const wcstring &var_name) {
+    (void)op;
+    (void)var_name;
+    int new_width = 1;
+    if (auto width_str = env_get(L"fish_ambiguous_width")) {
+        new_width = fish_wcstol(width_str->as_string().c_str());
+    }
+    g_fish_ambiguous_width = std::max(0, new_width);
+}
+
 static void handle_term_size_change(const wcstring &op, const wcstring &var_name) {
     UNUSED(op);
     UNUSED(var_name);
@@ -860,6 +870,7 @@ static void setup_var_dispatch_table() {
     var_dispatch_table.emplace(L"fish_term24bit", handle_fish_term_change);
     var_dispatch_table.emplace(L"fish_escape_delay_ms", handle_escape_delay_change);
     var_dispatch_table.emplace(L"fish_emoji_width", handle_change_emoji_width);
+    var_dispatch_table.emplace(L"fish_ambiguous_width", handle_change_ambiguous_width);
     var_dispatch_table.emplace(L"LINES", handle_term_size_change);
     var_dispatch_table.emplace(L"COLUMNS", handle_term_size_change);
     var_dispatch_table.emplace(L"fish_complete_path", handle_complete_path_change);

src/fallback.cpp

@@ -249,6 +249,10 @@ int killpg(int pgr, int sig) {
 }
 #endif
 
+// Width of ambiguous characters. 1 is typical default.
+int g_fish_ambiguous_width = 1;
+
+// Width of emoji characters.
 int g_fish_emoji_width = 0;
 
 // 1 is the typical emoji width in Unicode 8.
@@ -289,8 +293,10 @@ int fish_wcwidth(wchar_t wc) {
             // Fall back to system wcwidth in this case.
             return wcwidth(wc);
         case widechar_ambiguous:
+            return g_fish_ambiguous_width;
         case widechar_private_use:
-            return 1;
+            // TR11: "All private-use characters are by default classified as Ambiguous".
+            return g_fish_ambiguous_width;
         case widechar_widened_in_9:
             return fish_get_emoji_width(wc);
         default:

src/fallback.h

@@ -16,6 +16,9 @@
 // substitution if wchar.h is included after this header.
 #include <wchar.h>  // IWYU pragma: keep
 
+/// The column width of ambiguous East Asian characters.
+extern int g_fish_ambiguous_width;
+
 /// The column width of emoji characters. This must be configurable because the value changed
 /// between Unicode 8 and Unicode 9, wcwidth() is emoji-ignorant, and terminal emulators do
 /// different things. See issues like #4539 and https://github.com/neovim/neovim/issues/4976 for how