msgstore: do not build threads when threaded view is off

Commit 3a614e45fc ("threading: enable toggle-threads for server-side
threads") changed the behavior of the msgstore.buildThreads variable to
reflect whether the client needs to build threads or the server will.
However, a call to runThreadbuilder was not updated with an extra
conditional. As a result, threads were built regardless of the state of
the threadedView resulting in a large performance penalty for
non-threaded views with client side threading.

Only run thread builder if threaded view is enabled.

Fixes: 3a614e45fc ("threading: enable toggle-threads for server-side threads")
Reported-by: akspecs <akspecs@gmail.com>
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Akspecs <akspecs@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

lib/msgstore.go

@@ -356,7 +356,7 @@ func (store *MessageStore) update() {
 	if store.onUpdateDirs != nil {
 		store.onUpdateDirs()
 	}
-	if store.BuildThreads() {
+	if store.BuildThreads() && store.ThreadedView() {
 		store.runThreadBuilder()
 	}
 }