swev-id: matplotlib__matplotlib-24570 fix HPacker top/bottom alignment

[casey-brooks]

Reproduction steps

1. Use the script from [Bug] HPacker align 'top'/'bottom' reversed (Task 270 — matplotlib__matplotlib-24570) #38 (reproduced below) and set align="bottom" or align="top" on the HPacker.

import matplotlib.pyplot as plt
from matplotlib.offsetbox import DrawingArea, HPacker, VPacker, AnchoredOffsetbox, TextArea
from matplotlib.patches import Rectangle


da1 = DrawingArea(10, 20)
rect1 = Rectangle((0, 0), 10, 20)
da1.add_artist(rect1)

da2 = DrawingArea(10, 30)
rect2 = Rectangle((0, 0), 10, 30)
da2.add_artist(rect2)

align = "bottom"

pack = HPacker(children=[da1, da2], pad=10, sep=10, align=align)
title = TextArea(f"align='{align}'")
pack = VPacker(children=[title, pack], sep=10, pad=10, align="center")

box = AnchoredOffsetbox(child=pack, loc="center")

_, ax = plt.subplots()
ax.add_artist(box)

Observed failure

• align='bottom' aligns the top edges and align='top' aligns the bottom edges, so the visual result is inverted relative to the API contract.

Expected behavior

• align='top' should flush the children's top edges and align='bottom' should flush their bottom edges.

Summary of changes

• Swap the HPacker top/bottom mapping in _get_aligned_offsets so offsets make top alignment flush the top edges and bottom alignment flush the bottom edges.
• Clarify the HPacker docstring and add geometry-based regression tests that cover top, bottom, center, and baseline alignment along with a VPacker left/right sanity check.
• Add a behavior release note fragment describing the bugfix.

Testing

• pytest -k offsetbox
• flake8 lib/matplotlib/offsetbox.py lib/matplotlib/tests/test_offsetbox_alignment.py

Fixes #38.

[casey-brooks]

Tests & Lint Summary

• pytest -k offsetbox (276 passed, 8 skipped)
• flake8 lib/matplotlib/offsetbox.py lib/matplotlib/tests/test_offsetbox_alignment.py (no issues)

[noa-lucent]

Swapping the HPacker top/bottom handling fixes the inversion while leaving center and baseline math unchanged. Verified VPacker offsets are stable and the new geometry-based tests cover top/bottom/center/baseline as well as a left/right sanity check. Release note fragment is in the right location with clear wording. Looks good.