fix for split cells

rich/segment.py

@@ -129,34 +129,31 @@ def _split_cells(cls, segment: "Segment", cut: int) -> Tuple["Segment", "Segment
 
         cell_size = get_character_cell_size
 
-        pos = int((cut / cell_length) * (len(text))) - 1
-        if pos < 0:
-            pos = 0
+        pos = int((cut / cell_length) * len(text))
 
-        before = text[:pos]
-        cell_pos = cell_len(before)
-        if cell_pos == cut:
-            return (
-                _Segment(before, style, control),
-                _Segment(text[pos:], style, control),
-            )
-        while pos < len(text):
-            char = text[pos]
-            pos += 1
-            cell_pos += cell_size(char)
+        while True:
             before = text[:pos]
-            if cell_pos == cut:
+            cell_pos = cell_len(before)
+            out_by = cell_pos - cut
+            if not out_by:
                 return (
                     _Segment(before, style, control),
                     _Segment(text[pos:], style, control),
                 )
-            if cell_pos > cut:
+            if out_by == -1 and cell_size(text[pos]) == 2:
+                return (
+                    _Segment(before[:pos] + " ", style, control),
+                    _Segment(" " + text[pos + 1 :], style, control),
+                )
+            if out_by == +1 and cell_size(text[pos]) == 2:
                 return (
                     _Segment(before[: pos - 1] + " ", style, control),
                     _Segment(" " + text[pos:], style, control),
                 )
-
-        raise AssertionError("Will never reach here")
+            if cell_pos < cut:
+                pos += 1
+            else:
+                pos -= 1
 
     def split_cells(self, cut: int) -> Tuple["Segment", "Segment"]:
         """Split segment in to two segments at the specified column.

tests/test_segment.py

@@ -288,11 +288,16 @@ def test_split_cells_emoji(text, split, result):
 def test_split_cells_mixed() -> None:
     """Check that split cells splits on cell positions."""
     # Caused https://github.com/Textualize/textual/issues/4996 in Textual
-    test = Segment("早乙女リリエル (CV: 徳井青）")
-    for position in range(1, test.cell_length):
-        left, right = Segment.split_cells(test, position)
-        assert cell_len(left.text) == position
-        assert cell_len(right.text) == test.cell_length - position
+    tests = [
+        Segment("早乙女リリエル (CV: 徳井青）"),
+        Segment("メイド・イン・きゅんクチュアリ☆"),
+        Segment("TVアニメ「メルクストーリア -無気力少年と瓶の中の少女-」 主題歌CD"),
+    ]
+    for test in tests:
+        for position in range(1, test.cell_length):
+            left, right = Segment.split_cells(test, position)
+            assert cell_len(left.text) == position
+            assert cell_len(right.text) == test.cell_length - position
 
 
 def test_split_cells_doubles() -> None: