Generate <Shift-Button> event on a canvas (on Windows)

Hello out there,
I am writing tests (using tcltest) to avoid any regression during
development but I hit something like a brick wall: 'event generate'
appears not to work on items of a canvas. On regular widgets no
problem:  It can produce events like <Button-1>, <Shift-Button-1> ,
etc.
With twapi on the other hand I can produce <Button-1> events
everywhere: canvas, widgets - but no <Shift-Button-1>, so I'm stuck.
So the question is: How to make 'event generate' work on canvas items
or how to tweak twapi to also produce <Shift-Button> events.
Any help will be greatly apppreciated
Helmut
PS: I am running Tcl 8.6.10 under Windows 10, 64 bit