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, 66 insertions, 0 deletions

diff --git a/docs/superpowers/plans/2026-08-03-post-0.1.0-usability-closed.md b/docs/superpowers/plans/2026-08-03-post-0.1.0-usability-closed.md
index f062106..55c5ee7 100644
--- a/docs/superpowers/plans/2026-08-03-post-0.1.0-usability-closed.md
+++ b/docs/superpowers/plans/2026-08-03-post-0.1.0-usability-closed.md
@@ -9249,3 +9249,69 @@ mail was repaired by hand the same day. The regression test builds a thread
 whose messages DISAGREE about the tag, since two messages in the same state
 answer identically whichever way the code resolves them, which is the trap item
 87 already records.
+
+## 178. Delete and Restore judge a conversation on one message
+
+**Done 2026-08-29**, unreleased, on `thread-row-identity`. Split out of item
+168 by item 177 and flagged in that spec's "Open, deliberately".
+
+**Observed.** `MainWindow::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 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.** Delete and Archive are ABSENT on a
+reply row (item 177, at the user's own decision) and Restore is thread-scoped,
+so every path through this application is all-or-nothing. Two things outside it
+produce one: another client trashing a single message, which is live rather
+than hypothetical since the user runs Thunderbird (item 104), and a reply
+arriving after the conversation was trashed, which needs no other client at
+all.
+
+**Cause and fix.** The summary carries one path because the query walk stops at
+one message, and it stops there deliberately: the Sent branch's own measurement
+records that walk as free only because it breaks at the first match. Collecting
+every path on every query would make 36,000 rows pay for a question about the
+one the user clicked.
+
+`ThreadDigest` already walks every message of the selected conversation, for
+the 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:
+`ThreadDigest::messagePaths`, relative to the mail root for the reason
+`firstMessagePath` records, filled in `loadThreadDigest()` and consumed by
+`onThreadDigestLoaded()`. The predicate tests every path for a conversation row
+and one path for a message row.
+
+**One limit, stated 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 being
+given a second, differently wrong rule of its own. Making it exhaustive costs a
+per-query walk over every message, which is what this fix exists to avoid.
+
+**Tests.** Two, both mutation-checked.
+
+- `aDigestCarriesEveryMessagePath` puts the two messages in DIFFERENT folders,
+  since two in one folder answer identically whichever way the code resolves
+  them (item 87's rule). It also asserts the paths are relative, because an
+  absolute one matches no account and silently resolves every row to none.
+- `aPartlyTrashedConversationIsNotJudgedOnOneMessage` asserts both directions:
+  Delete survives and Restore hides on a partly trashed conversation, and the
+  wholly trashed case still answers as it always did, which is what says the
+  fix narrowed nothing. Reverting the predicate fails it with the reported
+  symptom.
+
+The second test sets `totalCount` explicitly and asserts `isConversationRow()`
+before proceeding. A summary left at the default is a MESSAGE row, so a test
+meaning to exercise a conversation would quietly exercise the other branch and
+pass for the wrong reason, which is the fixture trap the top-level document
+records.