csi: implement private mode 2031 (dark/light mode detection)

* Recognize 'CSI ? 996 n', and respond with
  - 'CSI ? 997 ; 1 n' if the primary theme is active
  - 'CSI ? 997 ; 2 n' if the alternative theme is actice
* Implement private mode 2031, where changing the color
  theme (currently only possible via key bindings) causes the terminal
  to send the same CSI sequences as above.

In this context, foot's primary theme is considered dark, and the
alternative theme light (since the default theme is dark).

Closes #2025

CHANGELOG.md

@@ -71,6 +71,11 @@
   `key-bindings.color-theme-toggle` key bindings. These can be used to
   switch between the primary and alternative color themes. They are
   not bound by default.
+* Support for private mode 2031 - [_Dark and Light Mode
+  Detection_](https://contour-terminal.org/vt-extensions/color-palette-update-notifications/)
+  ([#2025][2025])
+
+[2025]: https://codeberg.org/dnkl/foot/issues/2025
 
 
 ### Changed

csi.c

@@ -563,6 +563,10 @@ decset_decrst(struct terminal *term, unsigned param, bool enable)
 #endif
         break;
 
+    case 2031:
+        term->report_theme_changes = enable;
+        break;
+
     case 2048:
         if (enable)
             term_enable_size_notifications(term);
@@ -657,6 +661,7 @@ decrqm(const struct terminal *term, unsigned param)
     case 2027: return term->conf->tweak.grapheme_width_method != GRAPHEME_WIDTH_DOUBLE
         ? DECRPM_PERMANENTLY_RESET
         : decrpm(term->grapheme_shaping);
+    case 2031: return decrpm(term->report_theme_changes);
     case 2048: return decrpm(term->size_notifications);
     case 8452: return decrpm(term->sixel.cursor_right_of_graphics);
     case 737769: return decrpm(term_ime_is_enabled(term));
@@ -702,6 +707,7 @@ xtsave(struct terminal *term, unsigned param)
     case 2004: term->xtsave.bracketed_paste = term->bracketed_paste; break;
     case 2026: term->xtsave.app_sync_updates = term->render.app_sync_updates.enabled; break;
     case 2027: term->xtsave.grapheme_shaping = term->grapheme_shaping; break;
+    case 2031: term->xtsave.report_theme_changes = term->report_theme_changes; break;
     case 2048: term->xtsave.size_notifications = term->size_notifications; break;
     case 8452: term->xtsave.sixel_cursor_right_of_graphics = term->sixel.cursor_right_of_graphics; break;
     case 737769: term->xtsave.ime = term_ime_is_enabled(term); break;
@@ -746,6 +752,7 @@ xtrestore(struct terminal *term, unsigned param)
     case 2004: enable = term->xtsave.bracketed_paste; break;
     case 2026: enable = term->xtsave.app_sync_updates; break;
     case 2027: enable = term->xtsave.grapheme_shaping; break;
+    case 2031: enable = term->xtsave.report_theme_changes; break;
     case 2048: enable = term->xtsave.size_notifications; break;
     case 8452: enable = term->xtsave.sixel_cursor_right_of_graphics; break;
     case 737769: enable = term->xtsave.ime; break;
@@ -1539,6 +1546,32 @@ csi_dispatch(struct terminal *term, uint8_t final)
             break;
         }
 
+        case 'n': {
+            const int param = vt_param_get(term, 0, 0);
+
+            switch (param) {
+            case 996: {  /* Query current theme mode (see private mode 2031) */
+                /*
+                 * 1 - dark mode
+                 * 2 - light mode
+                 *
+                 * In foot, the themes aren't necessarily light/dark,
+                 * but by convention, the primary theme is dark, and
+                 * the alternative theme is light.
+                 */
+                char reply[16] = {0};
+                int chars = snprintf(
+                    reply, sizeof(reply),
+                    "\033[?997;%dn",
+                    term->colors.active_theme == COLOR_THEME1 ? 1 : 2);
+
+                term_to_slave(term, reply, chars);
+                break;
+            }
+            }
+            break;
+        }
+
         case 'p': {
             /*
              * Request status of ECMA-48/"ANSI" private mode (DECRQM

doc/foot-ctlseqs.7.scd

@@ -337,6 +337,9 @@ that corresponds to one of the following modes:
 |  2027
 :  contour
 :  Grapheme cluster processing
+|  2031
+:  contour
+:  Request color theme updates
 |  2048
 :  TODO
 :  In-band window resize notifications
@@ -657,6 +660,13 @@ manipulation sequences. The generic format is:
 :  xterm
 :  Report the current entry on the palette stack, and the number of
    palettes stored on the stack.
+|  \\E[ ? 996 n
+:  Query the current (color) theme mode
+:  contour
+:  The current color theme mode (light or dark) is reported as *CSI ?
+   997 ; 1|2 n*, where *1* means dark and *2* light. By convention, the
+   primary theme in foot is considered dark, and the alternative theme
+   light.
 
 
 # OSC

doc/foot.ini.5.scd

@@ -958,6 +958,10 @@ The colors are in RRGGBB format (i.e. plain old 6-digit hex values,
 without prefix). That is, they do *not* have an alpha component. You
 can configure the background transparency with the _alpha_ option.
 
+In the context of private mode 2031 (Dark and Light Mode detection),
+the primary theme (i.e. the *colors* section) is considered to be the
+dark theme (since the default theme is dark).
+
 *cursor*
 	Two space separated RRGGBB values (i.e. plain old 6-digit hex
 	values, without prefix) specifying the foreground (text) and
@@ -1093,6 +1097,10 @@ Note that values are not inherited. That is, if you set a value in
 *colors*, but not in *colors2*, the value from *colors* is not
 inherited by *colors2*.
 
+In the context of private mode 2031 (Dark and Light Mode detection),
+the primary theme (i.e. the *colors2* section) is considered to be the
+light theme (since the default theme is dark).
+
 # SECTION: csd
 
 This section controls the look of the _CSDs_ (Client Side

input.c

@@ -492,6 +492,9 @@ execute_binding(struct seat *seat, struct terminal *term,
             wayl_win_alpha_changed(term->window);
             term_font_subpixel_changed(term);
 
+            if (term->report_theme_changes)
+                term_to_slave(term, "\033[?997;1n", 9);
+
             term_damage_view(term);
             term_damage_margins(term);
             render_refresh(term);
@@ -506,6 +509,9 @@ execute_binding(struct seat *seat, struct terminal *term,
             wayl_win_alpha_changed(term->window);
             term_font_subpixel_changed(term);
 
+            if (term->report_theme_changes)
+                term_to_slave(term, "\033[?997;2n", 9);
+
             term_damage_view(term);
             term_damage_margins(term);
             render_refresh(term);
@@ -516,9 +522,15 @@ execute_binding(struct seat *seat, struct terminal *term,
         if (term->colors.active_theme == COLOR_THEME1) {
             term_theme_apply(term, &term->conf->colors2);
             term->colors.active_theme = COLOR_THEME2;
+
+            if (term->report_theme_changes)
+                term_to_slave(term, "\033[?997;2n", 9);
         } else {
             term_theme_apply(term, &term->conf->colors);
             term->colors.active_theme = COLOR_THEME1;
+
+            if (term->report_theme_changes)
+                term_to_slave(term, "\033[?997;1n", 9);
         }
 
         wayl_win_alpha_changed(term->window);

terminal.h

@@ -518,6 +518,7 @@ struct terminal {
 
     bool num_lock_modifier;
     bool bell_action_enabled;
+    bool report_theme_changes;
 
     /* Saved DECSET modes - we save the SET state */
     struct {
@@ -548,6 +549,7 @@ struct terminal {
         bool ime:1;
         bool app_sync_updates:1;
         bool grapheme_shaping:1;
+        bool report_theme_changes:1;
 
         bool size_notifications:1;