diff --git a/kitty/line.h b/kitty/line.h
index 684d3ea03..981324ceb 100644
--- a/kitty/line.h
+++ b/kitty/line.h
@@ -9,7 +9,6 @@
 
 #include "text-cache.h"
 
-// TODO: Test setting of ch_and_idx to make sure the right ch_is_idx bit is set
 // TODO: Test multiple OS windows with decorations also with changing font size and test on macOS and Linux for re-alloc
 // TODO: Test handling of calt ligatures with scale see is_group_calt_ligature()
 // TODO: Handle selection with multicell
diff --git a/kitty/screen.c b/kitty/screen.c
index 2e2508372..8c831d179 100644
--- a/kitty/screen.c
+++ b/kitty/screen.c
@@ -5260,6 +5260,20 @@ cpu_cells(Screen *self, PyObject *args) {
     return Py_NewRef(ans);
 }
 
+static PyObject*
+test_ch_and_idx(PyObject *self UNUSED, PyObject *val) {
+    CPUCell c = {0};
+    if (PyLong_Check(val)) {
+        unsigned long x = PyLong_AsUnsignedLong(val);
+        c.ch_and_idx = x;
+    } else if (PyTuple_Check(val)) {
+        c.ch_is_idx = PyLong_AsUnsignedLong(PyTuple_GET_ITEM(val, 0));
+        c.ch_or_idx = PyLong_AsUnsignedLong(PyTuple_GET_ITEM(val, 1));
+    }
+    unsigned long is_idx = c.ch_is_idx, idx = c.ch_or_idx, ca = c.ch_and_idx;
+    return Py_BuildValue("kkk", is_idx, idx, ca);
+}
+
 static PyMethodDef methods[] = {
     METHODB(test_create_write_buffer, METH_NOARGS),
     METHODB(test_commit_write_buffer, METH_VARARGS),
@@ -5411,6 +5425,7 @@ PyTypeObject Screen_Type = {
 static PyMethodDef module_methods[] = {
     {"is_emoji_presentation_base", (PyCFunction)screen_is_emoji_presentation_base, METH_O, ""},
     {"truncate_point_for_length", (PyCFunction)screen_truncate_point_for_length, METH_VARARGS, ""},
+    {"test_ch_and_idx", test_ch_and_idx, METH_O, ""},
     {NULL}  /* Sentinel */
 };
 
diff --git a/kitty_tests/multicell.py b/kitty_tests/multicell.py
index a9390ed5f..b74748f4b 100644
--- a/kitty_tests/multicell.py
+++ b/kitty_tests/multicell.py
@@ -2,7 +2,7 @@
 # License: GPLv3 Copyright: 2024, Kovid Goyal <kovid at kovidgoyal.net>
 
 
-from kitty.fast_data_types import TEXT_SIZE_CODE, wcswidth
+from kitty.fast_data_types import TEXT_SIZE_CODE, wcswidth, test_ch_and_idx
 
 from . import BaseTest, parse_bytes
 from . import draw_multicell as multicell
@@ -37,6 +37,13 @@ def test_multicell(self: TestMulticell) -> None:
             if assertions[key] != val:
                 raise AssertionError(f'{msg}{assertions[key]!r} != {val!r}')
 
+        self.ae(test_ch_and_idx(0), (0, 0, 0))
+        self.ae(test_ch_and_idx(1), (0, 1, 1))
+        self.ae(test_ch_and_idx(0x80000000), (1, 0, 0x80000000))
+        self.ae(test_ch_and_idx(0x80000001), (1, 1, 0x80000001))
+        self.ae(test_ch_and_idx((1, 0)), (1, 0, 0x80000000))
+        self.ae(test_ch_and_idx((1, 3)), (1, 3, 0x80000003))
+
         ae('x')
         ae('y')
         ae('width')