Fix off-by-one in code to nuke incomplete multicell chars in line

Fixes #8758

docs/changelog.rst

@@ -130,6 +130,8 @@ Detailed list of changes
 
 - Fix a regression in 0.40.0 that broke serialization of tab characters as ANSI text (:iss:`8741`)
 
+- Fix a regression in 0.40.0 that broke erasing of characters in a line in the presence of wide characters (:iss:`8758`)
+
 - kitten run-shell: Fix SIGINT blocked when execing the shell (:iss:`8754`)
 
 0.42.1 [2025-05-17]

kitty/screen.c

@@ -440,7 +440,7 @@ nuke_incomplete_single_line_multicell_chars_in_range(
         if (cpu_cells[x].is_multicell) {
             index_type mcd_x_limit = x + cpu_cells[x].width - cpu_cells[x].x;
             if (cpu_cells[x].x || mcd_x_limit > limit) nuke_in_line(cpu_cells, gpu_cells, x, MIN(mcd_x_limit, limit), replace_with_spaces ? ' ': 0);
-            x = mcd_x_limit;
+            x = mcd_x_limit - 1;
         }
     }
 }

kitty_tests/multicell.py

@@ -308,6 +308,12 @@ def test_multicell(self: TestMulticell) -> None:
 
     # Test delete chars with multicell (aka left shift)
     s.reset()
+    s.draw(' 允许')
+    s.cursor.x = 0
+    s.delete_characters(1)
+    for x in range(4):
+        ac(x, 0, width=2)
+    s.reset()
     multicell(s, 'a', width=2)
     s.cursor.x = 0
     s.delete_characters(1)