This is not the responsibility of findfont. It belongs in _findfont_cached.

The snapshot must be taken before lru_cache builds its key. Copying inside the decorated function still retains the original, reference-bearing object; I reproduced the figure remaining alive until the cache was cleared.

I kept this in findfont, the sole production caller, which already prepares the other cache-key inputs. An uncached private wrapper would also be correct, but would add another method and cache-invalidation changes without changing the supported behavior.

Fair point. It's a bit unfortunate that this is needed outside the cache, but the FontProperties conversion (prop = FontProperties._from_any(prop)) is inside, which we want to keep for speed.

I guess we have to live with the compromize of split responsibility.

I looked into the wrapper option a bit more and tried a small uncached wrapper around _findfont_cached. It keeps the snapshot before cache-key creation and the _from_any conversion inside the cached worker. Cache invalidation can also stay unchanged, so my earlier comment overstated the changes needed.
This still splits the two steps, but keeps the snapshot handling next to the cache implementation rather than in findfont. Would you prefer that small separation, or keeping the current placement?

What motivates the archtectural decision to reference self.axis.label? Have you considered other solution strategies?

If this approach is the right way forward it needs concise documentation what you do (semantically not technically) and why. Also, such a behavior should not be patched onto the label, but should be implmented as capability of the label.

self.axis.label already supplies the default text and color, so using it for the default font size follows the same behavior. Each axisartist label can still set its own size.

Overriding get_fontsize alone isn't enough here: Text uses _fontproperties directly for layout and drawing. Artist callbacks also don't catch changes made through get_fontproperties().set_size(…). That's why I kept the size lookup in FontProperties.

I agree that AxisLabel should own the setup. The local revision moves it there with set_fontsize("auto") and documents how setting a size or replacing the font properties stops inheritance. TickLabels keeps its existing behavior.

Technically, AxisLabel currently works with self.axis None. However, real use cases should always have self.axis set. If we could rely on that, would it be an option to always delegate font properties lookup to self.axis.label so that we don't have to maintain state in AxisLabel? This is turn would alleviate the need for _AxisLabelFontProperties.

I checked this against main. Left and right labels, as well as floating labels using the same Axis, can currently have independent font settings. Always using the reference label's FontProperties would make their setters modify the same object.

I'd prefer to preserve those local settings for this fix. Did you mean to delegate defaults while keeping local overrides, or to share font settings including edits? Keeping overrides would still require some per-label state, although it need not use _AxisLabelFontProperties.