Fix spacing in r"$\max f$".

Previously, in a mathtext string like `r"$\sin x$"`, a thin space would
(correctly) be added between "sin" and "x", but that space would be
missing in expressions like `r"$\max f$"`.  The difference arose because
of the slightly different handling of subscripts and superscripts
after the `\sin` and `\max` operators: `\sin^n` puts the superscript as
a normal exponent, but `\max_x` puts the subscript centered below the
operator name ("overunder symbol).  The previous code for inserting the
thin space did not handle the "overunder" case; fix that.  The new
behavior is tested by the change in test_operator_space, as well as by
mathtext1_dejavusans_06.

The change in mathtext_foo_29 arises because the extra thin space now
inserted after `\limsup` slightly shifts the centering of the whole
string.  Ideally that thin space should be suppressed if there's no
token after the operator, but that's not something currently implemented
either for e.g. `\sin` (compare e.g. the right-alignments in
`text(.5, .9, r"$\sin$", ha="right"); text(.5, .8, r"$\mathrm{sin}$", ha="right"); axvline(.5)`
where the extra thin space after `\sin` is visible), so this patch just
makes things more consistent.

Author: anntzer

lib/matplotlib/_mathtext.py

@@ -2362,8 +2362,7 @@ def operatorname(self, s: str, loc: int, toks: ParseResults) -> T.Any:
             next_char_loc += len('operatorname{}')
         next_char = next((c for c in s[next_char_loc:] if c != ' '), '')
         delimiters = self._delims | {'^', '_'}
-        if (next_char not in delimiters and
-                name not in self._overunder_functions):
+        if next_char not in delimiters:
             # Add thin space except when followed by parenthesis, bracket, etc.
             hlist_list += [self._make_space(self._space_widths[r'\,'])]
         self.pop_state()
@@ -2483,7 +2482,12 @@ def subsuper(self, s: str, loc: int, toks: ParseResults) -> T.Any:
                 shift = hlist.height + vgap + nucleus.depth
             vlt = Vlist(vlist)
             vlt.shift_amount = shift
-            result = Hlist([vlt])
+            result = Hlist([
+                vlt,
+                *([self._make_space(self._space_widths[r'\,'])]
+                  if self._in_subscript_or_superscript else []),
+            ])
+            self._in_subscript_or_superscript = False
             return [result]
 
         # We remove kerning on the last character for consistency (otherwise

lib/matplotlib/tests/test_mathtext.py

@@ -400,7 +400,7 @@ def test_operator_space(fig_test, fig_ref):
     fig_test.text(0.1, 0.6, r"$\operatorname{op}[6]$")
     fig_test.text(0.1, 0.7, r"$\cos^2$")
     fig_test.text(0.1, 0.8, r"$\log_2$")
-    fig_test.text(0.1, 0.9, r"$\sin^2 \cos$")  # GitHub issue #17852
+    fig_test.text(0.1, 0.9, r"$\sin^2 \max \cos$")  # GitHub issue #17852
 
     fig_ref.text(0.1, 0.1, r"$\mathrm{log\,}6$")
     fig_ref.text(0.1, 0.2, r"$\mathrm{log}(6)$")
@@ -410,7 +410,7 @@ def test_operator_space(fig_test, fig_ref):
     fig_ref.text(0.1, 0.6, r"$\mathrm{op}[6]$")
     fig_ref.text(0.1, 0.7, r"$\mathrm{cos}^2$")
     fig_ref.text(0.1, 0.8, r"$\mathrm{log}_2$")
-    fig_ref.text(0.1, 0.9, r"$\mathrm{sin}^2 \mathrm{\,cos}$")
+    fig_ref.text(0.1, 0.9, r"$\mathrm{sin}^2 \mathrm{\,max} \mathrm{\,cos}$")
 
 
 @check_figures_equal()