I have a null toolbar set (called 'No Toolbars Toolbar Set'), which contains no toolbars. Since my Opus interface is Tab driven, I'd like to use it for Tabs such as 'Music', to clear up the interface, and to provide more screen space.
I have no problem invoking this Toolbar Set using buttons or lister layouts, but I want it to trigger when I click on the 'Music' Tab. In preferences, I configured the 'List' file display mode to trigger this null toolbar set (using replace), but nothing happens. Other toolbars sets work fine.
Having looked into this in detail, the code is designed in a way that means an empty toolbar set is treated the same as specifying no toolbar set at all ("None").
We spent some time prototyping a change which would allow for an empty toolbar set to be distinct from not specifying a toolbar set, but unfortunately it didn't work very well and looked likely to introduce a lot of bugs if we continued with the idea, so we had to scrap it.
That said, you can still do what you wanted to do; it just has to be done the other way around.
First, save a toolbar set of your normal toolbar setup. In my example I called the set Leo's Menu & Operations since that's what was turned on in the set.
Then, configure Preferences / File Display Modes / Toolbars so that all of the display modes except Thumbnails specify that set, while Thumbnails specified None:
Finally, turn off all toolbars and then save that as the default toolbar set. (The easiest way to do that when no toolbars are visible is to type a > character into the file display, which will open up a command line at the bottom of the window, and then type Toolbar SETDEFAULT there and push return.)
Close the window and open a new one, and you should find it uses your normal toolbars in all the display modes except Thumbnails where it will have no toolbars at all, as desired.
(You may also want to turn on Preferences / Launching Opus / Default Lister / Ignore toolbars of Default Lister, and the similar option for any layouts which you use, to prevent them complicating matters.)
