Comment 47 for bug 885525

After fix of Bug 798663(status-thunderbird-esr17:fixed, landed on Tb 12.0.2) which resolved problem of Bug 815087, Tb 17.0.2 detects "Gmail IMAP or not" correctly, so problem of this bug(Trash) and a part of bug 800035(Archive) was exposed to users who are using Gmail IMAP for long time with mail.server.serverN.hostname!=imap.gmail.com.
(a) Trash case : bug 829185
    hostname=imap.googlemail.com case.
    By auto-config, hostname=imap.googlemail.com is used, perhaps by definition
    order in ISPDB.
(b) Archive case : bug 705491 comment #13
    Although "other than [Gmail]/All Mail" can be selected at UI and is used,
    "Archives Options..." is disabled and "archive to .../yy/mm" is impossible.

"Real" solution for them is "they can continue to use Tb without big change in their daily operation".

Because "Gmail IMAP detection" is sorted out by bug 798663(now based on X-GM-EXT-1 obtained by CAPABILITY response upon each login), there is no need to keep is_gmail flag in prefs.js any more.

If meaning/usage of is_gmail in prefs.js will be changed,
  (a) If no X-GM-EXT-1, clear is_gmail in prefs.js.
      (mandatory to bypass problem like Bug 815087)
  (b) If X-GM-EXT-1 is detected, never write/update is_gmail of prefs.js.
  (c) If is_gmail=false of prefs.js is set by user,
      override(clear) higher level "Gmail IMAP indicator" which is inherited
      from kGmailImapCapability bit of eIMAPCapabilityFlag.
they can use "trash other than [Gmail]/Trash" without any other change in Tb including correct solution of bug 800035.
And, they can archive mail to "yy/mm under [Gmail]/All Mail" and "yy/mm under Archives" even with Gmail IMAP.

This is another way to make "Forcing [Gmail]/Trash as trash of Tb if Gmail IMAP" optional.

As for Trash, there is no need to copy deleted mail to Trash in IMAP(\Deleted flag is used), and there is no reason to hate "Just mark it as deleted". So, I don't think such change is mandatory and has highest priority.
However, for users who used Tb & Gmail IMAP with "other than [Gmail]/Trash as Tb's trash" for long time, with undestanding Gmail IMAP well, this bug is a regression and such change is mandatory for their convenience.