Bug summary

Title.

Code for reproduction

import matplotlib.pyplot as plt
# plt.rcdefaults()
fig = plt.figure()
for (x, fontset) in enumerate(['cm', 'dejavusans', 'dejavuserif', 'stix', 'stixsans']):
    plt.rc('mathtext', fontset=fontset)
    fig.text(x / 5, .4, r'|$\,$|', size=100, alpha=.4)
    fig.text(x / 5, .6, r'|$\mathrm{\,}$|', size=100, alpha=.4)
    fig.text(x / 5, .9, fontset)
plt.show()

Actual outcome

\, and \mathrm{\,} do not insert the same amount of space.

Expected outcome

\, and \mathrm{\,} should insert the same amount of space. This is what LaTeX does (as far as I can tell by eye).

Additional information

The difference is too minute to matter. As the MWE shows, the text has to be blown up to huge sizes to manifest it. At normal sizes, it is inconsequential. (Case in point: test_mathtext.py::test_operator_space, which compares \sin^2 \cos with \mathrm{sin}^2 \mathrm{\,cos}, thereby indirectly comparing \, and \mathrm{\,}.)

It can have unexpected side-effects, though; see #22852 (comment).

The culprit is the _make_space function. For \,, it returns 1/6 of the advance of an italicised 'm'. For \mathrm{\,}, it returns 1/6 of the advance of a regular 'm'.

Making the following change does not break any Mathtext tests. (On my system. Though 120 tests get skipped, so I can't be sure.)

diff --git a/lib/matplotlib/_mathtext.py b/lib/matplotlib/_mathtext.py
index 3c4be21445..c630249282 100644
--- a/lib/matplotlib/_mathtext.py
+++ b/lib/matplotlib/_mathtext.py
@@ -1941,7 +1941,7 @@ class Parser:
         width = self._em_width_cache.get(key)
         if width is None:
             metrics = state.font_output.get_metrics(
-                state.font, mpl.rcParams['mathtext.default'], 'm',
+                'it', mpl.rcParams['mathtext.default'], 'm',
                 state.fontsize, state.dpi)
             width = metrics.advance
             self._em_width_cache[key] = width

Is it worth fixing?

Operating system

Linux Mint 20.2 Uma

Matplotlib Version

3.5.0.dev5379+g37ccdca502

Matplotlib Backend

GTK3Agg

Python version

3.8.10

Jupyter version

6.0.3

Installation

git checkout