Enable 24-bit RGB colors by default

I think `set_color ff0000` should default to outputting true-color sequences.
Unfortunately there is no good and widely-supported way to query for true-color
support.  `COLORTERM=truecolor` doesn't work in some cases such as ssh.

Since many terminals nowadays implement the RGB sequences, let's try using
them by default.

Note that Emacs's ansi-term implements truecolor now.

See also the discussion around
https://github.com/fish-shell/fish-shell/pull/11345#issuecomment-2794920900

Closes #11372

CHANGELOG.rst

@@ -26,6 +26,11 @@ Deprecations and removed features
     set -Ua fish_features no-ignore-terminfo
 
 - The `--install` option when fish is built as self-installable was removed. If you need to write out fish's data you can use the new `status list-files` and `status get-file` subcommands, but it should no longer be necessary. (:issue:`11143`)
+- RGB colors (``set_color ff0000``) now default to using 24-bit RGB true-color commands, even if the terminal does not advertise support via ``COLORTERM``.
+  - To go back to using the nearest match from the 256-color palette, use ``set fish_term24bit 0`` or ``set COLORTERM 0``.
+    - To make the nearest-match logic use the 16 color palette instead, use ``set fish_term256 0``.
+  - Inside macOS Terminal.app, fish makes an attempt to still use the palette colors.
+    If that doesn't work, use ``set fish_term24bit 0``.
 
 Scripting improvements
 ----------------------

doc_src/cmds/set_color.rst

@@ -22,7 +22,13 @@ Valid colors include:
 
 The *br*- (as in 'bright') forms are full-brightness variants of the 8 standard-brightness colors on many terminals. **brblack** has higher brightness than **black** - towards gray.
 
-An RGB value with three or six hex digits, such as A0FF33 or f2f can be used. Fish will choose the closest supported color. A three digit value is equivalent to specifying each digit twice; e.g., ``set_color 2BC`` is the same as ``set_color 22BBCC``. Hexadecimal RGB values can be in lower or uppercase. Depending on the capabilities of your terminal (and the level of support ``set_color`` has for it) the actual color may be approximated by a nearby matching reserved color.
+An RGB value with three or six hex digits, such as A0FF33 or f2f can be used.
+A three digit value is equivalent to specifying each digit twice; e.g., ``set_color 2BC`` is the same as ``set_color 22BBCC``.
+Hexadecimal RGB values can be in lower or uppercase.
+
+If :envvar:`fish_term24bit` is set to 0, fish will translate RGB values to the nearest color on the 256-color palette.
+If :envvar:`fish_term256` is also set to 0, fish will translate them to the 16-color palette instead.
+Fish launched as ``fish -d term_support`` will include diagnostic messages that indicate the color support mode in use.
 
 A second color may be given as a desired fallback color. e.g. ``set_color 124212 brblue`` will instruct set_color to use *brblue* if a terminal is not capable of the exact shade of grey desired. This is very useful when an 8 or 16 color terminal might otherwise not use a color.
 
@@ -72,17 +78,3 @@ Examples
     set_color blue; echo "Violets are blue"
     set_color 62A; echo "Eggplants are dark purple"
     set_color normal; echo "Normal is nice" # Resets the background too
-
-
-Terminal Capability Detection
------------------------------
-
-Fish uses some heuristics to determine what colors a terminal supports to avoid sending sequences that it won't understand.
-
-In particular it will:
-
-- Enable 24-bit ("true-color") even if the $TERM entry only reports 256 colors. This includes modern xterm, VTE-based terminals like Gnome Terminal, Konsole and iTerm2.
-
-To force true-color support on or off, set :envvar:`fish_term24bit` to "1" for on and 0 for off - ``set -g fish_term24bit 1``.
-
-Fish launched as ``fish -d term_support`` will include diagnostic messages that indicate the color support mode in use.

doc_src/language.rst

@@ -1566,12 +1566,12 @@ You can change the settings of fish by changing the values of certain variables.
 
 .. envvar:: fish_term24bit
 
-   If this is set to 1, fish will assume the terminal understands 24-bit RGB color sequences, and won't translate them to the 256 or 16 color palette.
-   This is often detected automatically.
+   If this is set to 0, fish will not output 24-bit RGB true-color sequences but the nearest color on the 256 color palette (or the 16 color palette, if :envvar:`fish_term256` is 0).
 
 .. envvar:: fish_term256
 
-   If this is set to 0, fish will not output 256 colors, but translate colors down to the 16 color palette.
+   If this is set to 0 and :envvar:`fish_term24bit` is 0, translate RGB colors down to the 16 color palette.
+   Also, if this is set to 0, :doc:`set_color <cmds/set_color>`/` commands such as ``set_color ff0000 red`` will prefer the named color.
 
 .. envvar:: fish_ambiguous_width
 

src/env_dispatch.rs

@@ -382,6 +382,7 @@ fn update_fish_color_support(vars: &EnvStack) {
 
     let term = vars.get_unless_empty(L!("TERM"));
     let term = term.as_ref().map_or(L!(""), |term| &term.as_list()[0]);
+    let is_xterm_16color = term == "xterm-16color";
 
     let supports_256color = if let Some(fish_term256) = vars.get(L!("fish_term256")) {
         let ok = crate::wcstringutil::bool_from_string(&fish_term256.as_string());
@@ -392,10 +393,10 @@ fn update_fish_color_support(vars: &EnvStack) {
         );
         ok
     } else {
-        term != "xterm-16color"
+        !is_xterm_16color
     };
 
-    let mut supports_24bit = false;
+    let supports_24bit;
     if let Some(fish_term24bit) = vars.get(L!("fish_term24bit")).map(|v| v.as_string()) {
         // $fish_term24bit
         supports_24bit = crate::wcstringutil::bool_from_string(&fish_term24bit);
@@ -408,19 +409,14 @@ fn update_fish_color_support(vars: &EnvStack) {
                 "disabled"
             }
         );
-    } else if vars.get(L!("STY")).is_some() || term.starts_with(L!("eterm")) {
-        // Screen and emacs' ansi-term swallow true-color sequences, so we ignore them unless
-        // force-enabled.
+    } else if vars.get(L!("STY")).is_some() {
+        // Screen requires "truecolor on" to enable true-color sequences, so we ignore them
+        // unless force-enabled.
         supports_24bit = false;
-        FLOG!(
-            term_support,
-            "True-color support: disabled for eterm/screen"
-        );
+        FLOG!(term_support, "True-color support: disabled for screen");
     } else if let Some(ct) = vars.get(L!("COLORTERM")).map(|v| v.as_string()) {
         // If someone sets $COLORTERM, that's the sort of color they want.
-        if ct == "truecolor" || ct == "24bit" {
-            supports_24bit = true;
-        }
+        supports_24bit = ct == "truecolor" || ct == "24bit";
         FLOG!(
             term_support,
             "True-color support",
@@ -432,32 +428,20 @@ fn update_fish_color_support(vars: &EnvStack) {
             "per $COLORTERM",
             ct
         );
-    } else if vars.get(L!("KONSOLE_VERSION")).is_some()
-        || vars.get(L!("KONSOLE_PROFILE_NAME")).is_some()
-    {
-        // All Konsole versions that use $KONSOLE_VERSION are new enough to support this, so no
-        // check is needed.
-        supports_24bit = true;
-        FLOG!(term_support, "True-color support: enabled for Konsole");
-    } else if let Some(it) = vars.get(L!("ITERM_SESSION_ID")).map(|v| v.as_string()) {
-        // Supporting versions of iTerm include a colon here.
-        // We assume that if this is iTerm it can't also be st, so having this check inside is okay.
-        if !it.contains(':') {
-            supports_24bit = true;
-            FLOG!(term_support, "True-color support: enabled for iTerm");
-        }
-    } else if term.starts_with("st-") {
-        supports_24bit = true;
-        FLOG!(term_support, "True-color support: enabling for st");
-    } else if let Some(vte) = vars.get(L!("VTE_VERSION")).map(|v| v.as_string()) {
-        if fish_wcstoi(&vte).unwrap_or(0) > 3600 {
-            supports_24bit = true;
-            FLOG!(
-                term_support,
-                "True-color support: enabled for VTE version",
-                vte
-            );
-        }
+    } else {
+        supports_24bit = !is_xterm_16color
+            && !vars
+                .get_unless_empty(L!("TERM_PROGRAM"))
+                .is_some_and(|term| term.as_list()[0] == "Apple_Terminal");
+        FLOG!(
+            term_support,
+            "True-color support",
+            if supports_24bit {
+                "enabled"
+            } else {
+                "disabled"
+            },
+        );
     }
 
     let mut color_support = ColorSupport::default();