fix(text): raise a clear error instead of IndexError on glyph/char mismatch

manim/mobject/text/text_mobject.py

@@ -618,6 +618,18 @@ def font_size(self, font_val: float) -> None:
     def _gen_chars(self) -> VGroup:
         chars = self.get_group_class()()
         submobjects_char_index = 0
+        non_space_chars = sum(1 for char in self.text if not char.isspace())
+        if non_space_chars != len(self.submobjects):
+            raise ValueError(
+                f"Text {self.original_text!r} produced {len(self.submobjects)} "
+                f"glyph(s) for {non_space_chars} non-space character(s) with "
+                "disable_ligatures=True. This usually means the chosen font "
+                "implements some of its ligatures through an OpenType feature "
+                "that isn't disabled (e.g. 'calt', used for programming "
+                "ligatures like '<=' or '->' by fonts such as Fira Code), so "
+                "characters and glyphs no longer correspond one-to-one. Try a "
+                "different font to work around this."
+            )
         for char_index in range(len(self.text)):
             if self.text[char_index].isspace():
                 space = Dot(radius=0, fill_opacity=0, stroke_opacity=0)

tests/module/mobject/text/test_text_mobject.py

@@ -3,6 +3,8 @@
 from contextlib import redirect_stdout
 from io import StringIO
 
+import pytest
+
 from manim.mobject.text.text_mobject import MarkupText, Text
 
 
@@ -32,3 +34,19 @@ def warning_printed(font: str, **kwargs) -> bool:
 
     # check random string (should be warning)
     assert warning_printed("Manim!" * 3, warn_missing_font=True)
+
+
+def test_gen_chars_raises_clear_error_on_glyph_mismatch():
+    """``_gen_chars`` should raise a clear, actionable error instead of an
+    opaque ``IndexError`` when the number of rendered glyphs doesn't match
+    the number of non-space characters. This happens when a font implements
+    some of its ligatures (e.g. programming ligatures like ``<=``) through
+    an OpenType feature that ``disable_ligatures`` doesn't disable, such as
+    ``calt`` (see issue #3237). This test simulates that mismatch directly,
+    without depending on any particular font being installed.
+    """
+    text = Text("ab", disable_ligatures=True)
+    # Simulate a font that merged "ab" into a single ligature glyph.
+    text.submobjects = text.submobjects[:1]
+    with pytest.raises(ValueError, match="glyph"):
+        text._gen_chars()