Did you check that the examples this change was introduced to fix are not re-broken?

I have just checked, and all the examples mentioned in #5757 still work as expected.

The latest commit should fix the issue if facecolor or color is given (or an alias) but I see a few potential problems.

1. If you look at the documentation for BboxConnectorPatch you can see fill is also a valid kwarg argument. By always setting fill in the init, if a user actually passed in the fill argument see line 319 it would throw:
   TypeError: __init__() got multiple values for keyword argument 'fill'
   which does not seem like intended behaviour.
   The mark_inset function also suffers from this problem, which should also be able to take fill arguments see docs.

2. It seems like a bit of a maintenance issue to manually map the aliases on line 316 if they are ever updated or change, this would then break. Instead I would recommend handleing the kwargs like in matplotlib/axes/_axes.py Line 1971 and reference the _patch_alias_map directly (I know it has the _ private identifier but this seems consistent with how this is handled elsewhere in the existing code).

Suggested Fix:
For BboxConnectorPatch I would suggest something like:

from matplotlib.cbook import normalize_kwargs
import matplotlib.patches as Patches
.
.
.
kwargs = normalize_kwargs(kwargs, Patches._patch_alias_map)
if 'fill' in kwargs:
    Patch.__init__(self, **kwargs)
else:
    fill = ('facecolor' in kwargs) or ('color' in kwargs)
    Patch.__init__(self, fill=fill, **kwargs)

AND

For mark_inset something like:

kwargs = normalize_kwargs(kwargs, Patches._patch_alias_map)
if 'fill' in kwargs:
    pp = BboxPatch(rect, **kwargs)
else:
    fill = ('facecolor' in kwargs) or ('color' in kwargs)
    pp = BboxPatch(rect, fill=fill, **kwargs)

That way any valid input should be handled correctly.

@Mr-Ian-Ferguson could you put your suggested changes in a new pull request?

@Mr-Ian-Ferguson @dstansby What is the status of this?

I don't plan to pick this up, but thought I'd leave the PR open for reference.

Sorry I've been without internet for a while. I probably won't be picking this up again, just something I noticed in passing while looking for a different issue. That said I stand by my above suggestions and your welcome to use as you see fit; I just don't have time to do proper testing right now or do a full PR.

I still have no plans to pick this up, so going to close.