1) If I do not turn on confirmation for drag & drop, then when I drag and drop WinSCP automatically uses foreground transfer. Is there a way to turn on background transfer as default for all transfers?
Not yet. I may add it.
2) I tried to turn on "use same setting next time" and "transfer on background" on the copy confirmation window, but still background transfer is not automatically used for the next time. I have to select background transfer again.
"Use same settings next time" does not apply to "Transfer on background". That's why it is above it.
3) When I use background transfer, the program ask for password for each background transfer. This is unnecessary. The program could store the login password for later use in background.
I wanted to avoid having password in memory. Of course you may save your password to stored session. It will be used by background transfers as well.
As workaround, you may also load your stored session (without password) and enter password directly on login dialog, instead of password dialog during connection.
4) What is the option "Automatically popup prompts of background transfers when idle" in the Preference|Transfer|Background panel?
When turned on and background transfer needs your attention (error message, query, password prompt, etc), the window will popup. If you turn it off, it won't, just a flashing icon will be displayed next to transfer on queue list. You will need to double click it to show the prompt. I believe that it is useful when you have plenty of background transfers. Then it may be diffucult to distinguish what transfer does the prompt belong to, if it pops up automatically.
5) What is the "no confirmation" in the copy confirmation window?
It is basically to supress overwrite confirmations. The same as with Synchronize function.