[Bug]: Font fallback doesn't work on \mathdefault

[dedebenui]

Bug summary

Per character font fallback (#20740) works great, except in tick labels.

Code for reproduction

import matplotlib.pyplot as plt

# CMU fonts available here : https://www.1001fonts.com/download/cmu.zip
plt.rcParams["font.family"] = ["CMU Bright", "DejaVu Sans"]

s = "There are \u2212 \u00D7 in between!"
plt.figtext(0.18, 0.45, s)
plt.gcf().suptitle(s)
plt.gca().set_title(s)
plt.xscale("log")
plt.xlim(1e-2, 6e-2)
plt.show()

Actual outcome

In the tick labels, characters not covered by CMU Bright are rendered with a placeholder instead of falling back to DejaVu

Expected outcome

Tick labels should fall back to DejaVu for \u2212 (minus sign) and \u00D7 (times sign) as in the other text instances.

Additional information

It seems to happen only with tick labels, other instances of text rendering work as intended in #20740. I get the same outcome with both macosx and TkAgg backends

I get the following warnings:

Font 'default' does not have a glyph for '\u2212' [U+2212], substituting with a dummy symbol.
Font 'default' does not have a glyph for '\\times' [U+d7], substituting with a dummy symbol.

Operating system

macOS 14.3.1

Matplotlib Version

3.8.3

Matplotlib Backend

MacOSX

Python version

3.11.7

Jupyter version

No response

Installation

pip

[QuLogic]

This is some specific thing to do with mathtext which seems to some special glyph mapping itself, but the odd thing is that if you wrap the other text in $, that doesn't repeat the bug.

[QuLogic]

Ah, the difference is that the ticks wrap their text in \mathdefault which picks up CMU Bright that you set, but regular math text would use the fonts from the mathtext.* rcParams, so that's why the regular text appears to work even when wrapped in $.

So the bug is that in mathtext, \mathdefault text only uses the first font specified as the default. A shortened reproducer is:

import matplotlib.pyplot as plt

# CMU fonts available here : https://www.1001fonts.com/download/cmu.zip
plt.rcParams["font.family"] = ["CMU Bright", "DejaVu Sans"]

s = "$\\mathdefault{\u2212 \u00D7 \\times}$"
plt.figtext(0.5, 0.5, s, fontsize=30, horizontalalignment='center')
plt.show()

which "works" if you replace \mathdefault with \mathrm.

[dedebenui]

It seems that mathdefault also ignores axes.unicode_minus:

import matplotlib.pyplot as plt

# CMU fonts available here : https://www.1001fonts.com/download/cmu.zip
plt.rcParams["font.family"] = ["CMU Bright", "DejaVu Sans"]
plt.rcParams["axes.unicode_minus"] = False
s = "$\\mathdefault{\u2212 \u00D7 \\times - } - $ - "
plt.figtext(0.5, 0.5, s, fontsize=30, horizontalalignment='center')
plt.show()