A new protocol extension to unscroll the screen

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

docs/changelog.rst

@@ -7,6 +7,10 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 0.20.2 [future]
 ----------------------
 
+- A new protocol extension to :ref:`unscroll <unscroll>` text from the
+  scrollback buffer onto the screen. Useful, for example, to restore
+  the screen after showing completions below the shell prompt.
+
 - Linux: Fix binary kitty builds not able to load fonts in WOFF2 format
   (:iss:`3506`)
 

docs/protocol-extensions.rst

@@ -157,6 +157,37 @@ protocol extension, it can be disabled by specifying ``no-append`` to the
 :opt:`clipboard_control` setting.
 
 
+.. _unscroll:
+
+Unscrolling the screen
+-----------------------
+
+This is a small extension to the `SD (Pan up) escape code
+<https://vt100.net/docs/vt510-rm/SD.html>`_ from the VT-420 terminal. The
+``SD`` escape code normally causes the text on screen to scroll down by the
+specified number of lines, with empty lines appearing at the top of the screen.
+This extension allows the new lines to be filled in from the scrollback buffer
+instead of being blank.
+
+The motivation for this is that many modern shells will show completions in a
+block of lines under the cursor, this causes some of the on-screen text to be
+lost even after the completion is completed, because it has scrolled off
+screen. This escape code allows that text to be restored.
+
+The syntax of the escape code is identical to that of ``SD`` except that it has
+a trailing ``+`` modifier. This is legal under the `ECMA 48 standard
+<https://www.ecma-international.org/publications-and-standards/standards/ecma-48/>`_
+and unused for any other purpose as far as I can tell. So for example, to
+unscroll three lines, the escape code would be::
+
+    CSI 3 + T
+
+See `discussion here
+<https://gitlab.freedesktop.org/terminal-wg/specifications/-/issues/30>`_.
+
+.. versionadded:: 0.20.2
+
+
 .. _desktop_notifications:
 
 

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
+static void
-screen_reverse_scroll(Screen *self, unsigned int count) {
+_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) {
+    fill_from_scrollback = fill_from_scrollback && self->linebuf == self->main_linebuf;
-        count--;
+    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);

kitty_tests/parser.py

@@ -184,6 +184,9 @@ class TestParser(BaseTest):
         pb('\033[3 @', ('Shift left escape code not implemented',))
         pb('\033[3 A', ('Shift right escape code not implemented',))
         pb('\033[3;4 S', ('Select presentation directions escape code not implemented',))
+        pb('\033[1T', ('screen_reverse_scroll', 1))
+        pb('\033[T', ('screen_reverse_scroll', 1))
+        pb('\033[+T', ('screen_reverse_scroll_and_fill_from_scrollback', 1))
 
     def test_csi_code_rep(self):
         s = self.create_screen(8)

kitty_tests/screen.py

@@ -315,6 +315,12 @@ class TestScreen(BaseTest):
         def assert_lines(*lines):
             return self.ae(lines, tuple(str(s.line(i)) for i in range(s.lines)))
 
+        # test the reverse scroll function
+        s = prepare_screen(map(str, range(6)))
+        assert_lines('2', '3', '4', '5', '')
+        s.reverse_scroll(2, True)
+        assert_lines('0', '1', '2', '3', '4')
+
         # Height increased, width unchanged → pull down lines to fill new space at the top
         s = prepare_screen(map(str, range(6)))
         assert_lines('2', '3', '4', '5', '')