>>    gdk_frame_clock_paint_idle
>>    → gtk_container_idle_sizer
>>    → gtk_distribute_natural_allocation
>
> Why doesn't this process kick in after I shrink the frame width
> manually such that the menu bar is cropped?  Something in the course

It does for me. If I try to shrink the frame manually it either
pops back immediately or after a brief pause (occasionally
a long pause, but it usually "wakes up" when I interact with the UI).

>> Worst case scenario: If I grab the scrolled window class and mutilate it
>
> Above I meant using the gtk fixed window class for the container, not
> the scrolled window one.

Oh, sure - I wasn't clear - I tried adding a gtk fixed and it behaves
for our purposes the same way as an hbox - it honours resize requests,
despite its name. The fixed appears to refer to layout (positioning)
only, not size.