fix: judge a conversation's trash state on all of its messages

Item 178. everySelectedRowIsInATrashFolder() read
ThreadSummary::firstMessagePath for any row that was not a message row.
That was correct while a thread row MEANT that message (item 108) and
stopped being correct when item 177 made it mean the conversation. A
conversation is in the trash only when ALL of its messages are, so a
partly trashed thread answered on whichever message the query returned
first: Delete could be hidden on a conversation that still had mail
outside the trash, and Restore offered on one that mostly did not.

Not data-affecting. Both actions are no-ops in the wrong direction:
Delete on already-trashed mail takes moveMessages()' already-there
branch, and Restore on mail that was never trashed finds nothing to
move.

qtmaildir cannot produce such a thread itself, since Delete is absent on
a reply row and Restore is thread-scoped. Two things outside it can:
another client trashing a single message, and a reply arriving after the
conversation was trashed.

ThreadDigest already walks every message of the selected conversation
for its sender counts, and a filename is served from the index like
everything else in it, so the paths ride along on a request the
selection already makes rather than costing a walk on every query.
ThreadDigest::messagePaths is relative to the mail root, for the reason
firstMessagePath records: an absolute path matches no account and
silently resolves every row to none. MainWindow keeps them beside the
dashboard's thread id and clears them when the dashboard is left, so a
late digest cannot answer about another row.

One limit, stated in the code rather than hidden. The digest is
requested only for a single selected conversation row, so that is the
only case with a real answer; any other selection falls back to the
summary's one path. That fallback IS the pre-177 answer and is wrong in
exactly the same partial case, which is the point: a multi-row selection
is left no worse than it was, rather than given a second, differently
wrong rule of its own. Making it exhaustive costs a per-query walk over
every message, which is what this avoids.

Two tests, both mutation-checked. The worker test puts its two messages
in different folders, since two in one folder answer identically
whichever way the code resolves them. The window test asserts both
directions, so a fix that simply hid Delete everywhere would fail it,
and sets totalCount explicitly: a summary left at the default is a
message row, and the test would otherwise exercise the other branch and
pass for the wrong reason.

Suite: 42 of 43, with undoMovesTheMessageBack failing as it does on
master (item 136).

1 files changed, 10 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4bdc536..34d9d2e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -143,6 +143,16 @@ point at which they are stable.
 
 ### Fixed
 
+- **Delete and Restore now judge a whole conversation.** They asked whether a
+  row was in the trash by looking at one of its messages, so a conversation
+  with some messages trashed and some not answered on whichever the query
+  returned first: Delete could be hidden on a conversation that still had mail
+  outside the trash, and Restore offered on one that mostly did not. Both
+  actions were no-ops in the wrong direction rather than destructive. A
+  conversation is now in the trash only when every one of its messages is.
+  qtmaildir cannot produce such a thread itself, since it deletes and restores
+  whole conversations; another mail client trashing a single message can, and
+  so can a reply arriving after a conversation was trashed.
 - **Undo no longer rewrites messages the action never touched.** Undoing a
   thread-scoped action inverted its tags while keeping the whole thread as its
   scope, so undoing "mark thread read" on a conversation of 44 messages that