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The channel member list keeps a snapshot of the last roster it read, which it shows while the relay is disconnected. When two roster updates arrive close together, the member list starts a read for each. If the first read answered after the second, it still wrote its older roster over the snapshot. The live list stayed correct, but once the app went offline it showed the older roster: for example, a member just removed, or a bot role just changed, came back. A read that a later read has replaced no longer writes the snapshot. The snapshot cache numbers each read of a channel's roster, and only the latest read for that channel may write. A read that ends because nothing is watching the list any more (the user leaves the screen mid-read, or a one-off lookup such as sending a message) still writes, since its roster is the newest one. ### Validation New tests, which fail without this change: - Two roster updates arrive, the second read answers first and the first answers after it, with a roster, with no roster event, or with an empty roster. Once disconnected, the member list still shows the newer roster. - A roster update arrives and the user leaves before its read answers. Once disconnected, the member list shows that newer roster. (This passes on main and guards against fencing on disposal, which an earlier version of this PR did.) The rest of the channel management tests pass unchanged. Full mobile test suite and `flutter analyze` pass. The race itself needs two roster reads to answer out of order and then a disconnect, which a device run can't reliably produce. An automated run on an iPhone 12 mini signed in to Block's Builderlab (profile build of this head) covered the roster paths it touches: cold start, searching for and opening buzz-mobile, its details (86 members) and its member list (49 people, roles shown), scrolling it, backgrounding the app for 60 seconds, resuming, and reopening the member list, which loaded the same roster. ### Regressions None. A replaced read's result was already discarded for the live list; now it is also kept out of the offline snapshot. Reads that nothing replaced write the snapshot as before. --------- Signed-off-by: Tom Brow <tomb@squareup.com> Co-authored-by: Tom Brow <tomb@squareup.com> Co-authored-by: corey <2dafccf60ea6d431a568f6d67fc60016c1b2a51e415e09dc8b213b8e3606cf54@buzz.block.builderlab.xyz> Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz> Co-authored-by: perfie <1fc92b08616bef6025a30fa24528ba2417d2a0c82f2c67b62e7f912089235028@buzz.block.builderlab.xyz>