A new protocol extension to unscroll the screen

See https://gitlab.freedesktop.org/terminal-wg/specifications/-/issues/30

kitty/parser.c

@@ -932,7 +932,13 @@ dispatch_csi(Screen *screen, PyObject DUMP_UNUSED *dump_callback) {
             NO_MODIFIERS(end_modifier, ' ', "Select presentation directions escape code not implemented");
             CALL_CSI_HANDLER1(screen_scroll, 1);
         case SD:
-            CALL_CSI_HANDLER1(screen_reverse_scroll, 1);
+            if (!start_modifier && end_modifier == '+') {
+                CALL_CSI_HANDLER1(screen_reverse_scroll_and_fill_from_scrollback, 1);
+            } else {
+                NO_MODIFIERS(start_modifier, 0, "");
+                CALL_CSI_HANDLER1(screen_reverse_scroll, 1);
+            }
+            break;
         case DECSTR:
             if (end_modifier == '$') {
                 // DECRQM

kitty/screen.c

@@ -1136,17 +1136,29 @@ screen_reverse_index(Screen *self) {
     } else screen_cursor_up(self, 1, false, -1);
 }
 
-void
-screen_reverse_scroll(Screen *self, unsigned int count) {
+static void
+_reverse_scroll(Screen *self, unsigned int count, bool fill_from_scrollback) {
     // Scroll the screen down by count lines, not moving the cursor
     count = MIN(self->lines, count);
     unsigned int top = self->margin_top, bottom = self->margin_bottom;
-    while (count > 0) {
-        count--;
+    fill_from_scrollback = fill_from_scrollback && self->linebuf == self->main_linebuf;
+    while (count-- > 0) {
+        if (fill_from_scrollback) historybuf_pop_line(self->historybuf, self->alt_linebuf->line);
         INDEX_DOWN;
+        if (fill_from_scrollback) linebuf_copy_line_to(self->main_linebuf, self->alt_linebuf->line, 0);
     }
 }
 
+void
+screen_reverse_scroll(Screen *self, unsigned int count) {
+    _reverse_scroll(self, count, false);
+}
+
+void
+screen_reverse_scroll_and_fill_from_scrollback(Screen *self, unsigned int count) {
+    _reverse_scroll(self, count, true);
+}
+
 
 void
 screen_carriage_return(Screen *self) {
@@ -2888,6 +2900,15 @@ hyperlink_at(Screen *self, PyObject *args) {
     return Py_BuildValue("s", url);
 }
 
+static PyObject*
+reverse_scroll(Screen *self, PyObject *args) {
+    int fill_from_scrollback = 0;
+    unsigned int amt;
+    if (!PyArg_ParseTuple(args, "I|p", &amt, &fill_from_scrollback)) return NULL;
+    _reverse_scroll(self, amt, fill_from_scrollback);
+    Py_RETURN_NONE;
+}
+
 #define MND(name, args) {#name, (PyCFunction)name, args, #name},
 #define MODEFUNC(name) MND(name, METH_NOARGS) MND(set_##name, METH_O)
 
@@ -2911,6 +2932,7 @@ static PyMethodDef methods[] = {
     MND(hyperlinks_as_list, METH_NOARGS)
     MND(garbage_collect_hyperlink_pool, METH_NOARGS)
     MND(hyperlink_for_id, METH_O)
+    MND(reverse_scroll, METH_VARARGS)
     METHOD(current_char_width, METH_NOARGS)
     MND(insert_lines, METH_VARARGS)
     MND(delete_lines, METH_VARARGS)

kitty/screen.h

@@ -157,6 +157,7 @@ void screen_reverse_index(Screen *self);
 void screen_index(Screen *self);
 void screen_scroll(Screen *self, unsigned int count);
 void screen_reverse_scroll(Screen *self, unsigned int count);
+void screen_reverse_scroll_and_fill_from_scrollback(Screen *self, unsigned int count);
 void screen_reset(Screen *self);
 void screen_set_tab_stop(Screen *self);
 void screen_tab(Screen *self);