Could you confirm that bug or maybe I'm missing something...
 
1. Place a 1/4 midi note on a track using you keyboard or as manual entry
2. Open PRV and using SMART TOOL stretch that 1/4 note to a 400 beats note (yeah 400 beat... 100 measures!!!)
3. Release the mouse button and try to stretch that 400 beats note to a 401 beat note using the SMART TOOL...
4. The SMART TOOL is uneffective.... is that normal ?

That is a long note! 
BTW, I'd probably use the Event Inspector to set the duration (or the Event View) as that would be faster.  Mice only run so fast. 
I was able to repro the issue at certain zoom levels.  Once you have a really long note you need to zoom out in order to get the cursor for cropping. 

Yeah a long note because it sends midi data to Arkaos via loopmidi driver... A long midi note as the length of the song... I need it !

So you can reproduce it so it is clearly a bug. It is there since a long time but I neglect to write here... So now it is !

Interesting find.  :)
 
I DO urge you to consider what was noted above, just set the duration using Event View or Event Inspector, takes about no time at all to do that. WAY quicker, I would think.
 
Anyways, I hope that note isn't a vocal note, or your singer will be turning blue at some point :)
 
Bob Bone
 

Anybody can confirm... If so I will open a ticket...