Selecting a thread put you at the newest message. Anything unread above
that sat off the top of the pane with nothing to announce it, and the
only way to find out was to scroll up -- by which time the auto-mark-read
timer had marked the whole thread read anyway, so scrolling up meant
scrolling up to mail already counted as seen (#87).
Opening at the bottom is right when there is nothing to catch up on and
wrong the moment there is. The pane now opens on the oldest message that
was unread when the thread was opened, and falls back to the newest when
the thread has already been read.
mbunkus's out-of-order case is the one that rules out guessing at a
position. A participant whose server could not connect for hours
delivers a message long after it was written, and it lands in the middle
of a conversation that has already moved past it -- so "second to last",
or any other fixed offset from the end, finds nothing. Reading the
unread set is the only thing that does.
Two cases leave the pane where it is:
- a single message, which is already the whole pane
- the first unread being the first message, where the top of the pane
shows it anyway, together with the subject; scrolling to it would
push the subject off for nothing
It reads the set captured when the thread was opened rather than live
`$seen` state, for the same reason expansion does (#69): the mark-read
timer must not change the shape of what you are looking at. That also
makes the landing stable, because everything above the first unread
message is a collapsed row of fixed height -- nothing up there reflows
after the scroll.
The mock grows a thread that reproduces it: seven messages with the
unread one second, four more behind it. Verified against it. Opening the
thread lands the unread message flush against the top of the pane at
scrollTop 158; the old scroll to the newest message put it at 445, with
287px of the message -- header, sender and unread bar included -- above
the fold. On a thread whose first message is the unread one the pane
stays at 0 with the subject in view, where before it would have scrolled
333. Once the thread is read, reopening it goes back to the newest
message.
Opening a conversation with several unread messages showed them all
expanded, each with its unread bar. The moment the auto-mark-read timer
fired, every one of them collapsed except the last, and the bars
vanished -- so the messages you had just been given were taken away
again, and the only record of which ones they were went with them (#69).
Both came from the same place: expansion and the bar were derived from
`$seen`, live. Marking read on the server changed what the view thought
it was looking at.
Marking read is not the problem. Opening a thread is the signal that you
are reading it, and mbunkus was explicit that turning the setting off is
not the answer he wants. What was wrong was letting a change *this view
caused* alter its own shape underneath the reader.
The thread now remembers which messages were unread when it was opened,
and uses that for expansion and for the bar. The set only grows while a
thread is open -- a message arriving unread joins it -- and is discarded
on the way to another thread. The server still gets marked read on the
timer, exactly as before, and the message list still updates.
It is accumulated during render rather than in an effect. It is derived
purely from the messages already in hand and adding an id twice does
nothing, while an effect would repaint a frame later -- which is the
flicker this exists to remove.
Verified against the mock with markReadDelay at 0, the harshest setting,
where the timer fires immediately: six seconds after opening a
three-message thread, the server reports all three seen while the view
still shows all three expanded with their bars. Before, two of the three
would have collapsed in the first instant.
jcoffey-dev