#27858 Agent chat rail: the preview card opens at once, and a click lands at once

Leo, 2026-10-29:

"but how about fixing up the left side scroll rail thing (moving it to the left, giving it faster scrttoll/hover over) and being able to scroll it? and the various other noticeable ui or perf things"

Draft: measurements in progress. Full body follows.

Agent session: agent session

🤖 Generated with Claude Code

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Summary by cubic

Makes the agent chat rail instant: the preview card opens on the frame the pointer reaches a row, and a click lands the prompt at once with no glide (Leo, 2026-10-29: "faster scrttoll/hover over").

• Removes the 250 ms hover delay (CARD_DELAY is now 0); the card opens with the tick's magnification and follows the pointer row to row.
• A click on a tick, mark, or track scrolls the transcript in the frame of the click instead of gliding 200–400 ms.
• Bookmark jumps and Previous/Next Turn steps keep their smooth scroll.
• Browser tests pin the instant behavior: the hovered tick, its lit band, and the card sit on one center line with the card's left edge at the rail's right edge, and the WebKit motion test asserts every frame after a click already shows the landing.
• Adds a thread rail page to the latency harness measuring the hover card's open and a click's landing (the click targets a drawn prompt at least 300 px away, where a glide would run): hover actions arm the probe on pointerover, LATENCY_TRACE=1 prints each phase of a run to stderr, and the server banner is matched without its ANSI color escapes, across chunks, with a 60 s timeout instead of a silent hang.
• The click-jump latency action starts its window on the click event (the rail jumps on onClick) and targets only prompts whose landing scroll is in range.