Clarify cross_list wait flag documentation

Updates the drag helper comments to cover the truncated-list case,
where a same-list drag also takes the frame-reload response path
because the client omits the optimistic param.

Author: myabc

modules/backlogs/spec/support/pages/backlog.rb

@@ -560,7 +560,10 @@ def expect_no_filter_count(type)
     # A drag within one list is optimistic: the server answers 204 and no frame
     # reload happens, so the settle signal is the sortable-lists:moved event. A
     # cross-list drag still reloads the backlogs_container frame. Callers of
-    # drag_work_package(before:) crossing lists must pass cross_list: true.
+    # drag_work_package(before:) must pass cross_list: true whenever the move
+    # takes the frame-reload response path: crossing lists, or staying within a
+    # truncated list (the client omits the optimistic param there, so the
+    # server streams a reload).
     def drag_work_package(moved, before: nil, into: nil, cross_list: into.present?)
       raise ArgumentError, "You must specify either before or into" unless before.present? ^ into.present?
 
@@ -840,9 +843,11 @@ def wait_for_backlogs_turbo_stream(wait: Capybara.default_max_wait_time, frame_r
       end
     end
 
-    # A cross-list drag_work_package still reloads the backlogs_container frame;
-    # a same-list one is optimistic (204, no reload) and only ever settles via
-    # the sortable-lists:moved event.
+    # cross_list: true waits for the backlogs_container frame reload — used for
+    # genuine cross-list drags and for same-list drags in truncated lists, both
+    # of which take the stream-reload response. A plain same-list drag is
+    # optimistic (204, no reload) and only ever settles via the
+    # sortable-lists:moved event.
     def wait_for_drag_work_package(cross_list:, &)
       if cross_list
         wait_for_backlogs_turbo_stream(frame_reload: true, &)