Noted while reviewing documentation on the current behavoir in #29252.

Summary

Patches and Collections do not draw edges by default. This was introduced in #5596 for matplotlib 2.0. It seems that this was first achieved by a linewidth 0, but it this was changed to using an edgecolor of "none" while maintaining a finite linewidth.

As a consequence rcParams["patch.edgecolor"] does not have an effect by default. There are, however two cases when rcParams["patch.edgecolor"] becomes relevant:

1. If facecolor is "none", Patch and Collection take rcParams["patch.edgecolor"] as edgecolor.
   I assume, this is done to prevent the artist being completely invisible (no facecolor and no edgecolor).
2. If rcParams["patch.force_edgecolor"] is set to true, rcParams["patch.edgecolor"] is used instead of "none" as the

This is overall very confusing.

Ping @efiring, who introduced rcParams["patch.force_edgecolor"] in #6904. Can you comment on the background and whether the proposal below is a reasonable alternative?

Proposed fix

Can we switch to the following logic: patch.edgecolor is generally used as edgecolor and now defaults to "none" (to achieve the same behavior as before). To handle case 1. (prevent invisible artists) introduce a new rcParams["patch.edgecolor_fallback"] that has a special color only for that case.

Needed changes to default rcParams

matplotlibrc before:

#patch.edgecolor: black
#patch.force_edgecolor: False

matplotlibrc after

#patch.edgecolor: "none"            # default edgecolor for Patch and Collection
#patch.edgecolor_fallback: "black"  # edgecolor to be used if facecolor is "none" to prevent completely invisible artists

Needed changes to styles

Existing styles would migrate

• If patch.force_edgecolor was False:
  from:

  patch.edgecolor: COLOR
  #patch.force_edgecolor: False
  

  to

  #patch.edgecolor: "none"
  patch.edgecolor_fallback:  COLOR
  

• If patch.force_edgecolor was True:
  from:

  patch.edgecolor: COLOR
  patch.force_edgecolor: True
  

  to

  patch.edgecolor: COLOR
  patch.edgecolor_fallback:  COLOR
  

Transition path

• patch.force_edgecolor gets deprecated and raises a warning when used on rcParams.
• styles that don't use any of patch.edgecolor or patch.force_edgecolor do not have to do anything.
• styles that have set patch.edgecolor but not patch.force_edgecolor are a very edge case. They either have overlooked that the default force_edgecolor=False makes the setting ineffective, or they have intentionally only configured the somewhat arcane edge color fallback for non-filled artists. In the former case, they get "auto-fixed" through the new behavior. I'd argue that we can affort to not runtime-warn on the latter case (but if really wanted, one could add a check at least for loading stylesheets).