What toggles these icons on/off? I already have experienced that they will toggle on if enough resources are opened that all of the tabs (with text titles) can not be seen. However, sometimes the tabs toggle back to text, leaving me in a situation where I am unable to click a couple of tabs that are open.

Like this:

This happened after I added another Bible.
The only way I have found to get the icon view back is to float a panel, then close it.
Is this the currently normal function of this feature?