Hi,
in this thread, leo mentioned the possibility of toggling fileselection via the middle mouse-button.
But - I don't use the mouse with DOpus 
Is there a possibility to simulate the middle mouse-button with keyboard?
I need something like powermode/checkboxmode with the thumbnailmode, too.
Any ideas?
No, doesn´t seem so. I experimented with @nodeselect & "select next", "select prev", but of course this won´t work: @nodeselect refers to a previously used command or function, to reuse the seleted files for the next function, but not to change selection state all by itself.
A command like "select current nodeselect" would be nice for this task. The state would be visible by the normal color highlighting then. Or like in the the image viewer, where selecting/deselecting can be done via "ins" key. Maybe this should be also possible in non-viewer mode?!
I don't know if you've tried it or not but if you have checkbox turned on in details mode, pressing the spacebar toggles the checkbox on/off for highlighted files.
Yep, this works for the details mode; but I need it for the thumbnails mode 
Unfortunately I can't use checkboxmode there 
I send a feature request to GPsoft.