Allow other modules to alter menu_trail_by_path's parent candidates

Good idea. You should also provide hook_menu_trail_by_path_parent_candidates_alter() documentation in menu_trail_by_path.api.php file.
P.S. I would also provide a setting or hook to menus, which should work with menu_trail_by_path. but it's other issue.

See the patch.

See the patch.

Makes a lot of sense. Not sure about the naming/docs though - what does it mean to alter parent candidates? Does it change parents?

Yep, for me name "parent candidates" is also not obvious.
The module looks at the current menu path and explodes it by slash. Than it looks for alias to each path part.
In the module the result is called "an array of parent candidates".

Example: for path 'foo/bar/zee'
It will return array of internal Drupal paths for 'foo', 'foo/bar', 'foo/bar/zee'

Patch from #4 works great! Thanks!

Can we get this committed to dev! Great module, minor feature addition to make it super rad!

I used this patch to keep the first parent candidate from being selected rather than the last candidate. The module messed up navigation highlighting for one section of the site. The patch is great.

I used this patch over, https://www.drupal.org/node/2413925, because most of the navigation highlighting was working fine.

triggering automated testing

Thanks for contributing