navigate: Fix buggy detection of start/end.

I was not able to reproduce obviously badly broken behavior from these
logic bugs, but after the renaming of message_viewport helpers in the
last few commits, it's clear that this logic was trying to check if
we're actually at the start/end of the possibly message feed, not just
the rendered portion, and doing so incorrectly.

web/src/message_list_view.js

@@ -1451,6 +1451,15 @@ export class MessageListView {
         );
     }
 
+    is_start_rendered() {
+        // Used as a helper in checks for whether a given scroll
+        // position is actually the very start of this view. It could
+        // fail to be for two reasons: Either some older messages are
+        // not rendered due to a render window, or we haven't finished
+        // fetching the oldest messages for this view from the server.
+        return this._render_win_start === 0 && this.list.data.fetch_status.has_found_oldest();
+    }
+
     get_row(id) {
         const $row = this._rows.get(id);
 

web/src/navigate.js

@@ -100,7 +100,11 @@ export function page_down_the_right_amount() {
 }
 
 export function page_up() {
-    if (message_viewport.at_rendered_top() && !message_lists.current.visibly_empty()) {
+    if (
+        message_viewport.at_rendered_top() &&
+        !message_lists.current.visibly_empty() &&
+        message_lists.current.view.is_start_rendered()
+    ) {
         message_lists.current.select_id(message_lists.current.first().id, {then_scroll: false});
     } else {
         page_up_the_right_amount();
@@ -108,7 +112,11 @@ export function page_up() {
 }
 
 export function page_down() {
-    if (message_viewport.at_rendered_bottom() && !message_lists.current.visibly_empty()) {
+    if (
+        message_viewport.at_rendered_bottom() &&
+        !message_lists.current.visibly_empty() &&
+        message_lists.current.view.is_end_rendered()
+    ) {
         message_lists.current.select_id(message_lists.current.last().id, {then_scroll: false});
         unread_ops.process_visible();
     } else {

web/tests/example7.test.js

@@ -125,6 +125,7 @@ run_test("unread_ops", ({override}) => {
     // data setup).
     override(message_lists.current, "can_mark_messages_read", () => can_mark_messages_read);
     override(message_lists.current, "has_unread_messages", () => true);
+    override(message_lists.current.view, "is_end_rendered", () => true);
 
     // First, test for a message list that cannot read messages.
     can_mark_messages_read = false;
@@ -134,6 +135,12 @@ run_test("unread_ops", ({override}) => {
 
     // Now flip the boolean, and get to the main thing we are testing.
     can_mark_messages_read = true;
+    // Don't mark messages as read until all messages in the narrow are fetched and rendered.
+    override(message_lists.current.view, "is_end_rendered", () => false);
+    unread_ops.process_visible();
+    assert.deepEqual(channel_post_opts, undefined);
+
+    override(message_lists.current.view, "is_end_rendered", () => true);
     unread_ops.process_visible();
 
     // The most important side effect of the above call is that