[Bug]: Inconsistent rendering between backends when rendering Mathtext horizontal rule

[tfpf]

Bug summary

When the last item in a Mathtext Vlist is an Hrule, the AGG and SVG backends render the image differently. The difference is not obvious at small font sizes.

To see this happen, we have to dive into Mathtext internals. An easy way is to modify fractions to end with the horizontal rule.

Code for reproduction

import matplotlib.pyplot as plt
import matplotlib._mathtext as _mathtext

plt.rcdefaults()

def _genfrac(self, ldelim, rdelim, rule, style, num, den):
    state = self.get_state()
    thickness = state.get_current_underline_thickness()
    vlist = _mathtext.Vlist([num, _mathtext.Vbox(0, 2 * thickness), _mathtext.Hrule(state, thickness)])
    return [_mathtext.Hlist([vlist])]
_mathtext.Parser._genfrac = _genfrac


(fig, ax) = plt.subplots()
plt.text(.5, .3, r'$\dfrac{up}{down}$', size=100, ha='center', va='baseline')
plt.axhline(.3)
plt.savefig('Hrule_at_bottom.png')
plt.savefig('Hrule_at_bottom.svg')
plt.show()

Actual outcome

PNG output

SVG output

Expected outcome

Both images should be identical.

Additional information

Why does it matter if it's related to a private module in Matplotlib?

This is one of the things holding up #23616. Vlists ending with an Hrule are not being positioned correctly.

Operating system

Ubuntu 20.04 LTS Focal Fossa on WSL

Matplotlib Version

3.5.0.dev5855+gee2f68bdc9

Matplotlib Backend

WXAgg

Python version

Python 3.8.10

Jupyter version

6.0.3

Installation

git checkout