fix(desktop): preserve the live tail when returning to a long Graph session

[me2seeks]

Problem

Returning to a long Graph-mode session while its latest turn is still running can restore the transcript one turn behind. The scroll container may end up numerically pinned to the bottom, but the virtual window does not include the live tail, so the viewport lands around the previous turn or contains no visible turn content.

Graph mode is enabled, with no operators created (0/0).

Reproduction

1. Enable Graph mode.
2. Build a conversation long enough to activate transcript virtualization.
3. Send another message and leave that turn running.
4. Switch to another session, then switch back.

Observed

On current main (a94d07732), an isolated Ubuntu Electron E2E reproduced this twice at 1440x900:

• before switching away: 15 projected turns, at the bottom;
• immediately after returning: 14 projected turns, 2128 px from the bottom;
• after arrival settling: the scroller reported bottom position, but no turn was visible and the original live-tail turn was not mounted.

This matches the UI landing around the second-to-last turn.

Expected

Returning to a running session keeps the current live turn in the transcript projection and lands on that turn at the bottom.

Scope and overlap checked

• perf(ui): stop the transcript from moving after a session switch #4105 changes transcript scroll ownership and arrival positioning, but does not preserve the missing live turn in the returned projection.
• fix(desktop): deliver mid-session tail append after loading history #4066 repairs durable tail delivery when a resident history window is away from the tail; this reproduction is the renderer's running-turn remount path.

A regression should extend the existing live-conversation remount E2E with long-history virtual-window and visible-tail assertions.

[me2seeks]

I'm working on it.

[me2seeks]

Extended verification corrected the initial diagnosis:

• the 15-to-14 count change is the bounded transcript range replacing its oldest loaded turn with the running tail, not a dropped live turn;
• on main, the running tail is mounted and visible after arrival settles;
• the brief intermediate jump/empty virtual window is the session-arrival scroll behavior already tracked by refactor(ui): give the transcript scroller a single position authority #4099 and addressed by perf(ui): stop the transcript from moving after a session switch #4105;
• the compact path also retained the latest turn in the verified fixture.

Closing this as a duplicate rather than opening an overlapping repair.