Stefan Monnier wrote:
> Calling the function just means "I'm a movement command and would like
> to follow the new select-on-shift behavior".
>   

It's a red herring to think in terms of movement commands.

For example, if C-t invokes transpose-chars then,
by default, S-C-t should also invoke transpose-chars
but override the default and make the new default to
preserve the transient mark.   S-C-f S-C-f S-C-t S-C-f
makes one modification to the buffer and leaves a
transient region of three characters.   (Hopefully
transpose-chars is already coded so as to make
its buffer changes under save-excursion.)



> Then the function can decide on what actually happens when.
>
>   

The default behavior is to remove the transient region.

The default treatment of a shift sequence with no other binding
is to run the unshifted sequence, but keep the transient region
(by default).

A tiny minority of commands need to behave differently
from that but that minority isn't identical to "motion commands"
and that minority isn't treated homogenously (e.g., yank and
replace-string are both exceptions but they are different kinds of
exceptions).





>>   "Extend the temporary region highlighting for the next command"
>>     
>
>   
>> is a neater and more self-contained task for a function than
>>     
>
>   
>>   "Extend the temporary region highlighting for the next command, but
>>    only if this-single-command-shift-translated is non-nil; otherwise,
>>    do nothing"
>>     
>
>   

That's true but it can be even simpler than that by
setting the defaults properly, with regard to the
right model of select-shift state.


> The docstring of the function would be more like
>
>   "Handle the mark selection for movement commands."
>
>   

"Movement commands" isn't the right concept.   The decision
to extend or not extend the transient region is more a matter of
how a command is invoked than what it does, usually.   (Again,
commands like yank are exceptions).

-t