Can you give me an idea on how to test this?

Sure!
So to reproduce the bug:

• I go to Terminator Preferences, select "Window geometry hints" and close Preferences
• try to resize the Terminator window

On all the distros I listed, it just ignores the geometry hints and resizes "smoothly".
Even if I restart Terminator, still same problem.

The problem should not be the config file, because it contains this:

[global_config]
  geometry_hinting = True
  ...

With my modifications:

• all Terminator instances must be closed (I guess it runs the older version otherwise)
• again, select "Window geometry hints" in Preferences
• I do python3 terminator to run it

It should work and resize correctly.

Also, some additional details: I tested the new version with the same config file and also after deleting the config file. It works on both cases.

About the disable_geometry_hints function:
without calling that function, if I go to Preferences and uncheck "Window geometry hints" I would need to restart Terminator to restore the "smooth" resize.

About using None instead of self:
I've read that around GNOME 3.20 the behavior changed in the C library. It seems it should replace every parameter with null. Maybe for a bug the python library does not do this correctly, I'm not sure.

Works. Pulling. Thank you so much!