msgstore: run threadBuilder with no debounce on DirectoryContents msg

Commit 54a0a377e0 ("threads: debounce client-side thread building")
introduced the option to debounce threadbuilding to improve performance
of client side threads. This caused a UI regression during filtering of
mailboxes:

1. The mailbox contains the full set of message UIDs
2. User filters mailbox
3. A DirectoryContents message is received from worker with the proper
   UIDs to display
4. Debounce is started (in a separate go routine)
5. The value of msgStore.Uids() is used to show pending headers, but is
   not updated until the threadbuilder runs. The full set of UIDs is
   shown briefly
6. Debounce is over, threadbuilder runs, updates msgStore.Uids(), proper
   messages show.

Run the thread builder immediately upon receipt of a DirectoryContents
message, bypassing any debounce. Performance will remain the same: the
debounce is meant to prevent multiple sequential calls to the thread
builder during scrolling. This is unlikely to occur in rapid succession
from filtering or sorting.

Fixes: https://todo.sr.ht/~rjarry/aerc/76
Fixes: 54a0a377e0 ("threads: debounce client-side thread building")
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

lib/msgstore.go

@@ -208,7 +208,7 @@ func (store *MessageStore) Update(msg types.WorkerMessage) {
 		}
 		store.Messages = newMap
 		store.uids = msg.Uids
-		update = true
+		store.runThreadBuilderNow()
 	case *types.DirectoryThreaded:
 		var uids []uint32
 		newMap := make(map[uint32]*models.MessageInfo)
@@ -368,18 +368,24 @@ func (store *MessageStore) BuildThreads() bool {
 }
 
 func (store *MessageStore) runThreadBuilder() {
-	if store.builder == nil {
-		store.builder = NewThreadBuilder()
-		for _, msg := range store.Messages {
-			store.builder.Update(msg)
-		}
-	}
 	if store.threadBuilderDebounce != nil {
 		if store.threadBuilderDebounce.Stop() {
 			logging.Infof("thread builder debounced")
 		}
 	}
 	store.threadBuilderDebounce = time.AfterFunc(store.threadBuilderDelay, func() {
+		store.runThreadBuilderNow()
+	})
+}
+
+// runThreadBuilderNow runs the threadbuilder without any debounce logic
+func (store *MessageStore) runThreadBuilderNow() {
+	if store.builder == nil {
+		store.builder = NewThreadBuilder()
+		for _, msg := range store.Messages {
+			store.builder.Update(msg)
+		}
+	}
 	// build new threads
 	th := store.builder.Threads(store.uids)
 
@@ -396,7 +402,6 @@ func (store *MessageStore) runThreadBuilder() {
 	if store.onUpdate != nil {
 		store.onUpdate(store)
 	}
-	})
 }
 
 // SelectedThread returns the thread with the UID from the selected message