The issue is this:
1. PathBrowser subclasses ModuleBrowser.
2. ModuleBrowser.init() has the common code for initializing both.
3. #31460 changed the signature of ModuleBrowser.__init__ *and* .init.
4. I must not have tested pathbrowser after the .init change.  Bad.
5. test_pathbrowser needs a new gui test class that at least calls PathBrowser.__init__, which will call ModuleBrowser.init

3.6.3 is OK, 3.7.0a2 has this bug.  3.7.0a3 is due 11/27.  Patch should be merged before this with or without new unittests.
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Module items can be expanded either by clicking [+] or double-clicking.  The latter also opens the file so that lines can be highlighted.  I have thought about disabling both, and opening a module browser instead, but I can see the usefulness of expanding several files one level deep to get an overview of parts or all of a package.  On the other hand, we might have double-click open both the module and a module browser.