Fix usetex path optical sizing - #32392

PR summary

Closes #32389

Problem

In usetex mode, TextToPath.get_glyphs_tex hardcoded LaTeX compilation to a fixed FONT_SCALE = 100 pt. Because TeX fonts have size-specific optical designs, TeX selected the large 17 pt design (CMSS17) and scaled it down, while layout measurement (TexManager.get_text_width_height_descent) typeset at the actual text size (e.g. CMSS8 for 6–8 pt).

This caused:

1. Font mismatch: SVG output used CMSS17 while PDF output used CMSS8.
2. Alignment errors: Right-aligned text with path_effects was narrower than its layout box and ended short of the alignment boundary.
3. TextPath(size=...) issue: TextPath accepted size separately without propagating it to prop.

Solution

• Updated TextToPath.get_glyphs_tex to typeset at the requested font size (fontsize = prop.get_size_in_points()) and rescale DVI coordinates and boxes back to FONT_SCALE units via scale = self.FONT_SCALE / fontsize.
• Normalizes prop via FontProperties._from_any(prop) for defensive input handling.
• Forwards explicit size in TextPath.__init__ to a copy of prop.
• Added unit tests in test_usetex.py covering optical font selection and TextPath size propagation.

Reproduction & Verification

import re
import matplotlib as mpl
import matplotlib.patheffects as pe
import matplotlib.pyplot as plt

mpl.rcParams["text.usetex"] = True
s = "Hamburgefonstiv"

# 1. 6 pt text saved as SVG and as PDF
fig = plt.figure(figsize=(2, 1))
fig.text(0.1, 0.5, s, fontsize=6)
fig.savefig("usetex.svg")
fig.savefig("usetex.pdf")
with open("usetex.svg", encoding="utf-8") as f:
    svg = f.read()
with open("usetex.pdf", "rb") as f:
    pdf = f.read()

print("SVG fonts:", sorted(set(re.findall(r'id="(CM[A-Z]+\d+)-', svg))))
print("PDF fonts:", sorted({m.decode() for m in re.findall(rb"/BaseFont\s*/(?:[A-Z]{6}\+)?(\w+)", pdf)}))

# 2. Right-aligned 8 pt text on Agg, without and with path effects
fig = plt.figure(figsize=(1.6, 0.5), dpi=600)
fig.add_artist(plt.Line2D([0.93, 0.93], [0, 1], lw=0.3, color="tab:red"))
fig.text(0.93, 0.62, s, fontsize=8, ha="right", va="center")
fig.text(0.93, 0.25, s, fontsize=8, ha="right", va="center", path_effects=[pe.Normal()])
fig.savefig("usetex_path_effects.png")

Expected Results with this PR:

• Output:

  SVG fonts: ['CMSS8']
  PDF fonts: ['CMSS8']
  

  (Before: SVG used ['CMSS17'])
• Image: In usetex_path_effects.png, both lines of text align flush with the red guide line.

Tests Run:

• pytest lib/matplotlib/tests/test_usetex.py -k "optical_sizing or textpath_size" (Passed)
• Regression suite: test_mathdefault, test_multiline_eqnarray, test_usetex_with_underscore, test_usetex_fallback, test_rotation, test_unicode_sizing (All passed)
• Code checks: git diff --check passed, no trailing whitespace or lint issues.

AI Disclosure

I used AI assistance to investigate the issue, trace relevant rendering paths, draft the implementation and tests, and help analyze test output. I reviewed the resulting changes, ran verification steps, and am responsible for the final contribution.

PR quality check

• Use an expressive title (e.g. Fix usetex path font optical sizing and alignment)
• New and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New features and API changes have release notes
• Documentation complies with general and docstring guidelines