If a student or teacher have got the permission to edit the content of a folder ressource (mod/folder:managefiles), the edit button does not appear in the view if the folder content is dispayed inline on the course page.

Presumed reproduction steps

• as teacher or manager, go to a course
  • turn editing on
• insert a folder ressource to the course, set "Display folder contents" to "Inline on a course page"
• enrol a student to the course
• for that folder, click edit → assign roles
• click role teacher and add the previously added student to this teacher role
• log out and log in as student
• scroll to the folder ressource and your cannot see a edit button even if you have the permission to edit (mod/folder:managefiles)

As I figured out the problem is the check on (moodle/course:manageactivities) for inline folder ressources.

So if you have the capability (moodle/course:manageactivities and mod/folder:managefiles) 

of a single inline folder ressourse activity you have no way to edit the content.

One Solution might be to remove the line of these checks on Line 84 in mod/folder/renderer.php:

 
        // Display the "Edit" button if current user can edit folder contents.
        // Do not display it on the course page for the teachers because there
        // is an "Edit settings" button right next to it with the same functionality.
        if (has_capability('mod/folder:managefiles', $context) &&
            ($folder>display != FOLDER_DISPLAY_INLINE || !has_capability('moodle/course:manageactivities', $context)))- {
            $editbutton = $this->output->single_button(
                new moodle_url('/mod/folder/edit.php', array('id' => $cm->id)),
                get_string('edit')
            );

            $buttons .= $editbutton;
        }