[Pidgin] #17405: Incorrect tab label color on system messages

[Pidgin]

#17405: Incorrect tab label color on system messages
--------------------+--------------------------
 Reporter:  kernc   |      Owner:  EionRobb
     Type:  defect  |     Status:  new
Milestone:  2.13.1  |  Component:  unclassified
  Version:  2.13.0  |   Keywords:
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 Since v2.13, chat window tabs get colored green (style "tab-label-typing",
 `#4e9a06`) instead of grey (style "tab-label-event", `#888a85`) even for
 system (e.g. join/part) messages (such as those from ''statenotify''
 plugin).

 Regression from v2.12. Tested with protocols [https://github.com/dequis
 /purple-facebook/ purple-facebook] and IRC.

 I sourced it to
 https://bitbucket.org/pidgin/main/src/60257ba57a28bc8cbae64385b3cf209c097c22de/pidgin/gtkconv.c?at=default#lines-5494:5497,
 implying the system message, or any one after it, has `PURPLE_IM_TYPING`
 flag set. 🤔

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Ticket URL: <https://developer.pidgin.im/ticket/17405>
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