Hidden next buttons

All of the above. 

I've worked with companies in highly regulated industries, and they need to be able to tell an auditor that anyone who completed a course was exposed to all of the content. That, of course, doesn't actually mean the person read it or understood it. But that's how compliance works...

The other debate is whether to change NEXT to Hidden or to Disabled.

• I disable it. Why? Because, when it's hidden, the PREV button moves into its place, which can be distracting and/or frustrating (e.g., if one clicks there expecting it to go next...). But I recognize that this is a matter of preference.

It is a really good question.

I have for years sat in the camp that you should always disable over hiding UI. 

The arguments for disabled buttons:

• If it is sequential flow where uses complete tasks in order they controls should be visible but disabled.
• It can reduce confusion as UI elements do not keep disappearing
• You can reduce the instructions as a disabled button already tells the user they need to do something
• It gives a sense of direction to the course as it pushes them forward as it unlocks 

The arguments against:

• Reduces cognitive load, users can focus more on the task and removes distractions.
• Simplicity,  saves space and allows us to change the controls at will
• Can cause confusion as users are trying to work out what the prerequisites are to unlock the next button
• Disabled controls contrast don't have to be accessible so may make them invisible
• users may not understand it is disabled and click it anyway

Sometimes it is better to test, I still go with disabled, but I have one caveat controls that never get enabled always get hidden, big.  the back button on the first slide and next on the last.

It depends on the content. If I don't want the learner to hit the Next button and skip through the content, then I add instructions at the beginning stating the Next button will appear once the content is completed for the slide. I create my own Next and Previous buttons. because the disabled or hidden Storyline next button confuse my learners.  So my Previous button will always be in the lower left corner of the screen, and my Next button will always appear in the lower right corner of the screen when the timeline finishes. I also add an audio (ding) when the Next button appears.

If the slide has content that needs to be clicked on or a video that needs to be viewed before the next button appears, then I add instructions to that slide explaining that. This is done using a voice over and then for safe measure, I add a visual note. My learners really get confused....very easily.