Add support for clearing the scrollback buffer

kitty now supports using \E3J to clear the screen and scrollback buffer.
See #268

kitty/graphics.c

@@ -728,9 +728,14 @@ clear_filter_func(ImageRef *ref, Image UNUSED *img, const void UNUSED *data) {
     return ref->start_row + (int32_t)ref->effective_num_rows > 0;
 }
 
+static inline bool
+clear_all_filter_func(ImageRef *ref UNUSED, Image UNUSED *img, const void UNUSED *data) {
+    return true;
+}
+
 void
-grman_clear(GraphicsManager *self) {
-    filter_refs(self, NULL, true, clear_filter_func);
+grman_clear(GraphicsManager *self, bool all) {
+    filter_refs(self, NULL, true, all ? clear_all_filter_func : clear_filter_func);
 }
 
 static inline bool

kitty/graphics.h

@@ -83,7 +83,7 @@ typedef struct {
 } ScrollData;
 
 GraphicsManager* grman_alloc();
-void grman_clear(GraphicsManager*);
+void grman_clear(GraphicsManager*, bool);
 const char* grman_handle_command(GraphicsManager *self, const GraphicsCommand *g, const uint8_t *payload, Cursor *c, bool *is_dirty);
 bool grman_update_layers(GraphicsManager *self, unsigned int scrolled_by, float screen_left, float screen_top, float dx, float dy, unsigned int num_cols, unsigned int num_rows);
 void grman_scroll_images(GraphicsManager *self, const ScrollData*);

kitty/history.c

@@ -90,6 +90,12 @@ historybuf_mark_line_dirty(HistoryBuf *self, index_type y) {
     self->line_attrs[index_of(self, y)] |= TEXT_DIRTY_MASK;
 }
 
+inline void
+historybuf_clear(HistoryBuf *self) {
+    self->count = 0;
+    self->start_of_data = 0;
+}
+
 static inline index_type
 historybuf_push(HistoryBuf *self) {
     index_type idx = (self->start_of_data + self->count) % self->ynum;

kitty/lineops.h

@@ -79,3 +79,4 @@ void historybuf_init_line(HistoryBuf *self, index_type num, Line *l);
 void historybuf_mark_line_clean(HistoryBuf *self, index_type y);
 void historybuf_mark_line_dirty(HistoryBuf *self, index_type y);
 void historybuf_refresh_sprite_positions(HistoryBuf *self);
+void historybuf_clear(HistoryBuf *self);

kitty/screen.c

@@ -103,7 +103,7 @@ void
 screen_reset(Screen *self) {
     if (self->linebuf == self->alt_linebuf) screen_toggle_screen_buffer(self);
     linebuf_clear(self->linebuf, BLANK_CHAR);
-    grman_clear(self->grman);
+    grman_clear(self->grman, false);
     self->modes = empty_modes;
 #define R(name) self->color_profile->overridden.name = 0
     R(default_fg); R(default_bg); R(cursor_color); R(highlight_fg); R(highlight_bg);
@@ -430,7 +430,7 @@ screen_handle_graphics_command(Screen *self, const GraphicsCommand *cmd, const u
 void
 screen_toggle_screen_buffer(Screen *self) {
     bool to_alt = self->linebuf == self->main_linebuf;
-    grman_clear(self->alt_grman);  // always clear the alt buffer graphics to free up resources, since it has to be cleared when switching back to it anyway
+    grman_clear(self->alt_grman, true);  // always clear the alt buffer graphics to free up resources, since it has to be cleared when switching back to it anyway
     if (to_alt) {
         linebuf_clear(self->alt_linebuf, BLANK_CHAR);
         screen_save_cursor(self);
@@ -882,6 +882,7 @@ screen_erase_in_display(Screen *self, unsigned int how, bool private) {
               including cursor position.
             * ``2`` -- Erases complete display. All lines are erased
               and changed to single-width. Cursor does not move.
+            * ``3`` -- Erase complete display and scrollback buffer as well.
         :param bool private: when ``True`` character attributes are left unchanged
     */
     unsigned int a, b;
@@ -891,7 +892,8 @@ screen_erase_in_display(Screen *self, unsigned int how, bool private) {
         case 1:
             a = 0; b = self->cursor->y; break;
         case 2:
-            grman_clear(self->grman);
+        case 3:
+            grman_clear(self->grman, how == 3);
             a = 0; b = self->lines; break;
         default:
             return;
@@ -911,6 +913,13 @@ screen_erase_in_display(Screen *self, unsigned int how, bool private) {
     if (how != 2) {
         screen_erase_in_line(self, how, private);
     }
+    if (how == 3 && self->linebuf == self->main_linebuf) {
+        historybuf_clear(self->historybuf);
+        if (self->scrolled_by != 0) {
+            self->scrolled_by = 0;
+            self->scroll_changed = true;
+        }
+    }
 }
 
 void