feat(vt): add support for DECST8C escape sequence

Recognize CSI ? 5 W as DECST8C, which resets the active screen's tab
stops to the default of every 8 columns. Other CSI W variants continue
to produce a parse error.

Signed-off-by: Ayman Bagabas <aymanbagabas@gmail.com>

docs/changelog.rst

@@ -176,6 +176,8 @@ Detailed list of changes
 0.47.1 [future]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+- Add support for the DECST8C escape sequence (``CSI ? 5 W``) to reset tab stops to every 8 columns
+
 - Preserve user-set tab stops across window resizes (previously they were reset to every 8 columns on every resize)
 
 - Fix a regression in the previous release that caused :ac:`copy_or_noop` to stop working correctly (:pull:`10041`)

kitty/screen.c

@@ -2075,6 +2075,11 @@ screen_set_tab_stop(Screen *self) {
         self->tabstops[self->cursor->x] = true;
 }
 
+void
+screen_reset_tab_stops(Screen *self) {
+    init_tabstops(self->tabstops, self->columns);
+}
+
 void
 screen_cursor_move(Screen *self, unsigned int count/*=1*/, int move_direction/*=-1*/, bool allow_move_to_previous_line) {
     if (count == 0) count = 1;
@@ -4978,6 +4983,7 @@ WRAP0(reverse_index)
 WRAP0(reset)
 WRAP0(set_tab_stop)
 WRAP1(clear_tab_stop, 0)
+WRAP0(reset_tab_stops)
 WRAP0(backspace)
 WRAP0(tab)
 WRAP0(linefeed)
@@ -6260,6 +6266,7 @@ static PyMethodDef methods[] = {
     MND(carriage_return, METH_NOARGS)
     MND(set_tab_stop, METH_NOARGS)
     MND(clear_tab_stop, METH_VARARGS)
+    MND(reset_tab_stops, METH_NOARGS)
     MND(start_selection, METH_VARARGS)
     MND(update_selection, METH_VARARGS)
     {"clear_selection", (PyCFunction)clear_selection_, METH_NOARGS, ""},

kitty/screen.h

@@ -245,6 +245,7 @@ void screen_set_tab_stop(Screen *self);
 void screen_tab(Screen *self);
 void screen_backtab(Screen *self, unsigned int);
 void screen_clear_tab_stop(Screen *self, unsigned int how);
+void screen_reset_tab_stops(Screen *self);
 void screen_set_mode(Screen *self, unsigned int mode);
 void screen_reset_mode(Screen *self, unsigned int mode);
 void screen_decsace(Screen *self, unsigned int);

kitty/vt-parser.c

@@ -1246,6 +1246,14 @@ dispatch_csi(PS *self) {
             }
             REPORT_ERROR("Unknown CSI R sequence with start and end modifiers: '%c' '%c' and %u parameters", start_modifier, end_modifier, num_params);
             break;
+        case 'W':
+            if (start_modifier == '?' && !end_modifier && num_params == 1 && params[0] == 5) {
+                REPORT_COMMAND(screen_reset_tab_stops);
+                screen_reset_tab_stops(self->screen);
+                break;
+            }
+            REPORT_ERROR("Unknown CSI W sequence with start and end modifiers: '%c' '%c' and %u parameters", start_modifier, end_modifier, num_params);
+            break;
         case ECH:
             CALL_CSI_HANDLER1(screen_erase_characters, 1);
         case DA:

kitty_tests/screen.py

@@ -530,6 +530,22 @@ def test_tab_stops(self):
         s = self.create_screen(cols=4, lines=2)
         s.draw('aaaX\tbbbb')
         self.ae(str(s.line(0)) + str(s.line(1)), 'aaaXbbbb')
+        # DECST8C: reset tab stops to every 8 columns
+        s = self.create_screen(cols=20, lines=2)
+        s.clear_tab_stop(3)
+        s.reset_tab_stops()
+        s.cursor_position(1, 1)
+        s.tab()
+        self.ae(s.cursor.x, 8)
+        s.tab()
+        self.ae(s.cursor.x, 16)
+        # Verify the DECST8C escape sequence
+        s = self.create_screen(cols=20, lines=2)
+        s.clear_tab_stop(3)
+        parse_bytes(s, b'\x1b[?5W')
+        s.cursor_position(1, 1)
+        s.tab()
+        self.ae(s.cursor.x, 8)
 
         # Custom tab stops survive a window resize
         s = self.create_screen(cols=20, lines=2)