Fix backspacing of wide characters in wide-character unaware programs not working

Fixes #875

docs/changelog.rst

@@ -13,6 +13,8 @@ Changelog
   font fallback for all subsequent characters that cannot be rendered in the
   main font to fail (:iss:`799`)
 
+- Fix backspacing of wide characters in wide-character unaware programs not working (:iss:`875`)
+
 - Linux: Fix number pad arrow keys not working when Numlock is off (:iss:`857`)
 
 - Wayland: Implement support for clipboard copy/paste (:iss:`855`)

kitty/screen.c

@@ -708,7 +708,15 @@ screen_is_cursor_visible(Screen *self) {
 
 void
 screen_backspace(Screen *self) {
-    screen_cursor_back(self, 1, -1);
+    unsigned int amount = 1;
+    if (self->cursor->x < self->columns && self->cursor->x > 1) {
+        // check if previous character is a wide character
+        linebuf_init_line(self->linebuf, self->cursor->y);
+        unsigned int xpos = self->cursor->x - 2;
+        GPUCell *gpu_cell = self->linebuf->line->gpu_cells + xpos;
+        if ((gpu_cell->attrs & WIDTH_MASK) == 2) amount = 2;
+    }
+    screen_cursor_back(self, amount, -1);
 }
 
 void

kitty_tests/screen.py

@@ -222,6 +222,15 @@ class TestScreen(BaseTest):
         for i in range(1, 5):
             self.ae(str(s.line(i)), '12345')
 
+    def test_backspace_wide_characters(self):
+        s = self.create_screen()
+        s.draw('⛅')
+        self.ae(s.cursor.x, 2)
+        s.backspace()
+        s.draw(' ')
+        s.backspace()
+        self.ae(s.cursor.x, 0)
+
     def test_resize(self):
         s = self.create_screen(scrollback=6)
         s.draw(''.join([str(i) * s.columns for i in range(s.lines)]))