msglist [nfc]: Say we'll show "loading" even when fetch is at other end

This is NFC ultimately because we currently only ever fetch, or
show loading indicators, at one end of the message list, namely
the start.

When we do start supporting a message list in the middle of history,
though (zulip#82), and consequently loading newer as well as older
messages, my conclusion after thinking it through is that we'll want
a "busy fetching" state at one end to mean we show a loading
indicator at the other end too, if it still has more to be fetched.

This would look weird if the user actually saw both at the same time
-- but that shouldn't happen, because if both ends (or even either
end) is still open then the original fetch should have found plenty of
messages to separate them, many screenfuls' worth.

And conversely, if the user does kick off a fetch at one end and then
scroll swiftly to the other end and witness how that appears, we want
to show them a "loading" sign.  The situation is exactly like if
they'd had a fetch attempt on that same end and we were backing off
from failure: there's no fetch right now, but there won't be one yet,
so effectively the loading is busy.

Author: gnprice

lib/widgets/message_list.dart

@@ -684,16 +684,21 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
   }
 
   Widget _buildStartCap() {
-    // These assertions are invariants of [MessageListView].
+    // If we're done fetching older messages, show that.
+    // Else if we're busy with fetching, then show a loading indicator.
+    //
+    // This applies even if the fetch is over, but failed, and we're still
+    // in backoff from it; and even if the fetch is/was for the other direction.
+    // The loading indicator really means "busy, working on it"; and that's the
+    // right summary even if the fetch is internally queued behind other work.
+
+    // (This assertion is an invariant of [MessageListView].)
     assert(!(model.fetchingOlder && model.fetchOlderCoolingDown));
-    final effectiveFetchingOlder =
+    final busyFetchingMore =
       model.fetchingOlder || model.fetchOlderCoolingDown;
-    assert(!(model.haveOldest && effectiveFetchingOlder));
-    return switch ((effectiveFetchingOlder, model.haveOldest)) {
-      (true, _) => const _MessageListLoadingMore(),
-      (_, true) => const _MessageListHistoryStart(),
-      (_,    _) => const SizedBox.shrink(),
-    };
+    return model.haveOldest ? const _MessageListHistoryStart()
+      : busyFetchingMore ? const _MessageListLoadingMore()
+      : const SizedBox.shrink();
   }
 
   Widget _buildEndCap() {