fix: message list being empty on chat double click

[WofWca]

• refactor: simplify jumpToMessage
• refactor: jumpToMessage more type safety, comments
• refactor: move var declaration in jumpToMessage
• fix: message list being empty on chat double click

The important commit is the last one. Reviewing commit-by-commit should be easier for full understanding.

To reproduce, simply double click on a chat in the chat list
really fast.
Or:

item = document.querySelector(
  '.chat-list-item:not(.archive-link-item):not(.selected)'
);
chatListItem.click();
// Let React re-render, then click again.
setTimeout(() => chatListItem.click());

2025-02-15-PC6uVIAd2n.mp4

The problem is, in selectChat we have code that is supposed to
jump to the last message when the currently selected chat
gets clicked again.
However, if the chat has not loaded yet, and thus a new instance
of MessageListStore has not been created yet, and thus its
loadChat() has not been called yet,
then, when loadChat() does get called eventually,
it will see that __internal_jump_to_message_asap is set,
and so it will call __jumpToMessage, with msgId: undefined.
But, it turns out, __jumpToMessage cannot handle this case.
It would try to look up the last message in
this.state.messageListItems, but it is empty
(see the items.length <= 0 line), so __jumpToMessage()
would simply return and do nothing, resulting in an empty chat.
But messageListItems is empty not because
there are really no messages in the chat,
but because MessageListStore has not loaded anything yet,
because, as you remember, loadChat() delegated its responsibility
to __jumpToMessage().

This commit ensures that __jumpToMessage() handles the case where
MessageListStore has not loaded anything yet,
and where msgId === undefined.

It appears that the reproduction steps above is the only way
this bug can occur, because it's pretty much the only way
to have msgId: undefined (except for the "jump down" button).
So this commit probably doesn't fix any other current bugs.

The issue that this commit fixes was likely introduced in
#4562
(637497c).

[nicodh]

Reviewed by testing & Reading & (mostly) understanding
;-)

[WofWca]

There is another bug that this MR fixes:

1. Open a chat
2. Open the "Gallery" tab.
3. Click on the same chat in the chat list.

The messages list will be empty.