<Session_name>: Transfer queue is empty

I'm using win7 64bit, winscp commander interface, select a lot of files and upload them by pressing F5.
Then I open a new session (let's call the first session A, the second B), select another files and start uploading them.
Now every time an upload finishes in session A (the hidden one), in the system tray a message bubble pops: "Session_name: Transfer queue is empty", however the queue isn't empty, the upload continues with the next file.
If I switch to session A, messages begin to appear from session B, every time a file upload is finished.

Well that's by design:
https://winscp.net/eng/docs/ui_pref_environment#notifications
What do you dislike about it?

The disturbing thing is the fact that winscp notifies me about an empty queue when a (one) file upload is finished, not when the entire queue is finished. So, if I upload 10 files, I got 10 messages "Transfer queue is empty". It should be only one message, after the 10th file...
Note again, this happens *only* when I have more sessions and the session in question is hidden (isn't focused)

There are separate queues for every session, that's why it says "<Session_name>: Transfer queue is empty". If it reports that queue is empty when there are files in the same queue, it ehould be bug. Is that true?

Yes.
Of course, I'm talking about the separate queues.

To simulate the bug, please start a session (let's call A), select several files (let's say 5) and start uploading. Now quickly start an another session (let's call B) and add another 4 files and start uploading.
Currently we see session B.
See what happens:
During the uploads, when the first file of session A finishes, a message pops up in the system tray: "A: Transfer queue is empty". When the second file finishes, another popup message.
If I switch to session A (B becomes invisible), I start receiving messages about session B.

This bug has been added to tracker. Thanks!