Fix an out of bounds memory access when drawing a multi line character with cursor below bottom margin

docs/changelog.rst

@@ -199,6 +199,12 @@ you use a decent Wayland compositor.
 Detailed list of changes
 -------------------------------------
 
+0.49.2 [future]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- Fix an out-of-bounds memory access when drawing a multi-line text sized
+  character with the cursor below the bottom margin of the scroll region
+
 0.49.1 [2026-09-24]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

kitty/screen.c

@@ -1568,11 +1568,19 @@ handle_fixed_width_multicell_command(Screen *self, CPUCell mcd, ListOfChars *lc)
     cell_set_chars(&mcd, self->text_cache, lc);
     move_cursor_past_multicell(self, width);
     if (height > 1) {
-        index_type available_height = self->margin_bottom - self->cursor->y + 1;
-        if (height > available_height) {
-            index_type extra_lines = height - available_height;
-            screen_scroll(self, extra_lines);
-            self->cursor->y -= extra_lines;
+        if (self->cursor->y <= self->margin_bottom) {
+            index_type available_height = self->margin_bottom - self->cursor->y + 1;
+            if (height > available_height) {
+                index_type extra_lines = height - available_height;
+                screen_scroll(self, extra_lines);
+                self->cursor->y -= extra_lines;
+            }
+        } else {
+            // Cursor is below the scroll region, which cannot be scrolled, so
+            // move the cursor up until the cell fits on screen. height <=
+            // max_height <= lines so this cannot underflow.
+            index_type available_height = self->lines - self->cursor->y;
+            if (height > available_height) self->cursor->y -= height - available_height;
         }
     }
     if (self->modes.mIRM) {

kitty_tests/multicell.py

@@ -25,6 +25,21 @@ class TestMulticell(BaseTest):
             s.draw('好好')  # two 2-cell wide chars covering columns 0..3
             s.test_draw_overlay_line('xy', xstart, 0)  # would SIGSEGV without fix
 
+    def test_multicell_below_bottom_margin(self):
+        # Regression: with the cursor below the bottom margin, the available
+        # height computation underflowed and cells were written past the end
+        # of the line buffer.
+        s = self.create_screen(cols=10, lines=6)
+        s.set_margins(1, 3)  # 1-based indexing
+        s.cursor.x, s.cursor.y = 0, s.lines - 1
+        multicell(s, 'A', scale=2)
+        self.ae((s.cursor.x, s.cursor.y), (2, s.lines - 2))
+        for y in (s.lines - 2, s.lines - 1):
+            for x in range(2):
+                c = s.cpu_cells(y, x)
+                self.assertIsNotNone(c['mcd'])
+                self.ae((c['x'], c['y'], c['mcd']['scale']), (x, y - s.lines + 2, 2))
+
 
 def test_multicell(self: TestMulticell) -> None:
     from kitty.tab_bar import as_rgb