Cannot send chat message to off-line contact.

Your contact likely set her presence to "invisible". To you it seems your contact is offline. Thus, you can assume a realtime chat conversation will not be possible. Hence, the only possibility is to send a normal message using an email style interface.

Allowing users to send chat messages to offline contacts does not seem to be a good idea to me. Some not so smart users would have difficulties understanding the concept of offline contacts. You might get annoying chat conversations to offline contacts like this:
15:00: Hi
15:02: How are you?
15:05: ????
15:07: Hello??!!
15:10: Why don't you answer me?

The email-like interface for messages of the type normal does not at all indicate there will be a realtime answer from the contact.

"I suggest to add a possibility to compose messages to the off-line contacts using a Chat window."

This is won't fix, but maybe you have another suggestion to improve the interface regarding this issue?

"This will also provide saving outgoing messages to off-line contacts into history, I believe (this bug has been reported before)."
Messages of the type normal should not appear in the conversation history. See: http://xmpp.org/rfcs/rfc3921.html#stanzas

  Hello!
  I'm glad, you have turned attention to this problem. :)

  You are saying there are some sort of "not very smart" users who may easily be confused by composing a massage to offline contact with a Chat window, am I right?
  Well, maybe, you are right. To be honest, I haven't seen such users yet, but I can imagine there are some. In such case, I suggest to make this feature OPTIONAL. I think that if those users are not very smart, they will not also change any default settings. There is no such a guarantee, of course, but I just think so. That is why making the current behaviour (composing Chat message in a Chat window and a Normal message - in a e-mail style window) default will save them from such a confusion. But we could discuss this question further and produce some kind of compromise solution.
  The thing is that now I'm talking from the point of view of a regular user. And from this point of view it is inconvenient, when, for example the following situation occurs:
  Imagine that I'm having a chat conversation with somebody. We are talking and talking and then I need to go away from my PC for some reason. The person I was talking with sends me a message and waits for reply, but I'm away and I cannot answer. After a while, I come back and see that that person has left the network. Just went off-line. But I still have and un-answered message in my chat window, which I cannot answer using the same chat window. Instead, I must use a separate window to compose and answer, which, by the way, will not be saved into history. But this message is still a chat message and it relates to what has been written above. Don't you think this is not very convenient?

  I can also give another example. It happens to me even more often.
  Example: I write a message to somebody. He is offline, so I have to use an e-mail-style compose window and send a "Normal" message. After some days has passed, this person appears on-line and answers to me. But I just do not remember, what I had written him/her several days ago. In such case I usually ask to give me my last message fo I could figure out what we are talking about.
  This happens to me very often, I must say. It really annoys.

  That is why I wish there were such an option to make all of my messages (both incoming and outgoing) chat-messages. They are always saved into history and they are always easier to answer to. It seems to be not a bad idea to place such option somewhere between the settings. :)

  And talking about "standards". Well, I do agree that following standards isn't a bad idea in general. But I also do believe that the program must first of all be convenient to users, and follow the standards just after that. I also do believe that standards should be convenient for users too. :)
  Or maybe it would be better to say, that a program must follow the users' wishes where it would not be a too big violation of standards. Talking about Coccinella, it may be in form of not "saving Normal messages into history", but "converting Normal messages into Chat messages and saving those Chat messages into history" (of course, if the user turns this option on by his own hand). I guess, this wouldn...

"After a while, I come back and see that that person has left the network. Just went off-line. But I still have and un-answered message in my chat window, which I cannot answer using the same chat window."

I checked this scenario and it looks you still can use the same open chat window to answer the unanswered message... So, I don't think there is a problem or did I misread something?

"After some days has passed, this person appears on-line and answers to me. But I just do not remember, what I had written him/her several days ago. In such case I usually ask to give me my last message fo I could figure out what we are talking about."

Agreed. What about integrating in some way the inbox to the chat history and the short history in the chat window? I think this will solve this inconvenience, right?

"It seems to be not a bad idea to place such option somewhere between the settings."

Additional options are *always* a bad idea if they can be avoided. Reason: they make the settings more complicated. So, that's why we first have to look if there is no better way to solve this usability issue without making the settings more complicated.

"But I also do believe that the program must first of all be convenient to users, and follow the standards just after that."

I don't agree. Standards are more important than convenience to users. Too much webmasters made the same mistake by first making their website as convenient as possible for the largest group of visitors (cfr. Internet Explorer). The right way is to start with clean HTML from the beginning.

"I'm not familiar with Tcl/Tk."

I'm still learning too :-)

How about:

leaving the double click action "as is". E.g. when it is configured to open a chat window, when double clicking on a contact, but the contact is offline, then the message window appears. This I would leave as is.

However, when right clicking on an offline contact, right now the chat is disabled. We could probably enable this one. Then maybe in the window head line, adding some text e.g. "WARNING: chat with offline contact"

And when a chat window is open with someone, and the other party goes offline, we could still allow sending chat messages to that contact, but when the contact goes offline/invisible, changing the window line also adding a warning "WARNING: contact went offline"

With regard to the history, I would leave everything as is, not mixing up message with chat history... (do we actually have a message history?)

That would mean: no extra configuration option. "dumb" users, (I at least assume they are able to read) "may" see and read the warning when chatting with an offline contact.

  Hello!

To sander:

""After a while, I come back and see that that person has left the network. Just went off-line. But I still have and un-answered message in my chat window, which I cannot answer using the same chat window."

I checked this scenario and it looks you still can use the same open chat window to answer the unanswered message... So, I don't think there is a problem or did I misread something?"

  -- Yes, there are both cases. Let me explain what I mean.
  If you have only ONE chat (if you are talking with only one person), you do have an ability to answer the messages even if that contact went off-line. BUT! If you have 2 or more tabs in a chat window, i.e. if you are talking with 2 or more people, you have such possibility only if the tab was active at the time the contact has left the network. So a contact leaves the network at the time you have a conversation with somebody else in another tab, you won't have an ability to use the same chat window to answer to off-line contact.
  If this explanation is not clear enough (I'm still learning English), please let me know.

"What about integrating in some way the inbox to the chat history and the short history in the chat window?"

-- I think that saving a "normal" messages history at least somewhere could solve this problem at some extent. But I'm not sure if this is also a violation of a standard or not.

"Additional options are *always* a bad idea if they can be avoided. "
  -- I don't agree with this statement, but I do not insist on adding any new options. I am sure such option would be convenient for me, but you are right that somebody would, probably, feel himself confused because of it. So let's not add any unnecessary options which would make the program uncomfortable to use for some users.

"I don't agree. Standards are more important than convenience to users"
 -- Well, from my own point of view, this is not always true. But again - I do not insist. If you think this is more important - let it be this way. And maybe you are completely right - after all it is a standard makers' work to make standards comfortable for users, and not yours. :)

To buzzdee:
"However, when right clicking on an offline contact, right now the chat is disabled. We could probably enable this one."
  -- Yeh, I think this could be a solution. At least a user may choose what kind of message he wants to send and sees clearly that a contact is offline and thus won't get his message immediately.
  But still I'm not sure about what kind of message can be sent to an off-line contact. Is it only "Normal message" or a "chat message" too? Won't sending a chat message to off-line user require some kind of conversion from "chat message" into "normal message" and sanding it in a form of normal message, but adding it into a chat history (because a chat window was used to compose it)?

"With regard to the history, I would leave everything as is, not mixing up message with chat history... (do we actually have a message history?)"
  -- As for me, I never use "Normal messages" directly. And my friends never do. We prefer to use a chat window for any conversation between us. But of course, if there are 2 kinds of...

@buzzdee: that seems to be a good solution.

PS: "do we actually have a message history?"-->yes, chat messages in the conversation history and normal messages in the inbox.

"...chat messages in the conversation history and normal messages in the inbox."
  Do we have an outgoing "Normal" messages history?

svn revision 2783 allows to send chat messages to offline contacts, here the svn log:
Implement wish as described in bug report: #514426
Cannot send chat message to off-line contact.
Now allows to send chat messages to offline (invisible)
contacts. Double clicking offline contacts always
opens the message window. The user explicitly needs
to right click and choose to chat to the contact.
In the window title, it states "Offline Chat:" instead
of "Chat:" now. When the contact goes offline
or comes online while chatting, then the title
gets updated when the focus of the window changes,
or tabs are switched.

There are still some other checks for the contacts availability.
Not sure whether those still make sense after the change.

So please test svn version or breakfast builds tomorrow and give feedback.

  Hello!
  Thanks for this fix. I'll test it tomorrow (March 01, 2010).

Regards,
Vladimir.

  Hello again!
  Tested. Works. At least, for now. I'll use this version for several days and if something terrible happens, I'll let you know. :)

  Thanks once more!

  Best wishes,
  Vladimir.