fix(trajectory): bound work during long streams

.agents/notes/implemented/feature/2026-07-27-trajectory-inspection-ledger.md

@@ -21,11 +21,11 @@ Trajectory has to make prose, machine payloads, token usage, timing, and nested
 - Call schemas come from the active recorded Request header. Keyless snapshot fixtures deliberately replace that catalog with the non-array `{{tools}}` token, which the durable inspection boundary treats as unavailable instead of attempting to project or fabricate schemas.
 - Selecting a record or Request opens an inspector inside Trajectory. Tabs and Summary sections follow the selected entity: Markdown messages expose rendered, source, provenance, and hierarchy views; tools add JSON payload/result and schema views; Requests add options, usage, timing, and result navigation. Images render as media rather than serialized data.
 - Turn folding removes all rows after its first record and replaces them with a compact step/tool-call count; Assistant folding applies the same interaction to its tool-call descendants. Global controls fold or expand both levels.
-- A long ledger initially positions the loaded tail at the bottom and mounts only the viewport's row window plus bounded overscan. Fixed row estimates and virtual spacer rows preserve the loaded scroll range, while selection, timeline focus, folding, search, and bottom following address records by stable event or tool-call identity rather than requiring their DOM rows to exist. An explicit loading row covers records until initial positioning finishes and while an older page is pending. Prepending that page restores the prior visible anchor instead of jumping to the new top; the raw window base sequence detects the prepend even when a page adds no surface-visible node.
+- A long ledger initially positions the loaded tail at the bottom and mounts only the viewport's row window plus bounded overscan. Request-only separators join the next measurable virtual item, with a terminal separator retaining its own fixed clearance, so the virtualizer never owns a zero-height item. Semantic DOM-safe row keys and ARIA indexes expose identity independently from mount position. A tail with known older history virtualizes immediately even when its loaded projection is below the ordinary row threshold. Stable-key virtualizer anchoring preserves the visible item across prepends and appends; the manual scroll-height fallback applies only when completing pagination disables virtualization. Selection, timeline focus, folding, search, and bottom following address records by stable event or tool-call identity rather than requiring their DOM rows to exist. An explicit loading row covers records until initial positioning finishes and while an older page is pending. The raw window base sequence detects a prepend even when a page adds no surface-visible node.
 - The separate Waterfall tab is removed. A fixed Overview above the ledger projects every loaded record with known `startedAt` onto three semantic timing lanes using its own duration. While an older prefix remains unloaded and the viewport includes the loaded domain's start, a neutral ellipsis control covers the truncated edge and loads one earlier page without assigning unknown history a fabricated duration; hovering that control suppresses the ordinary timeline cursor. Finalized Assistant spans divide the recorded interval at the first non-empty token delta, so distinct TTFT and decoding colors retain their actual ratio; incomplete timing falls back to one Assistant color. Hovering for 500 ms exposes exact start/end, total duration, TTFT, and decoding time without relying on the browser's native tooltip delay. Dragging left or right commits an inclusive interval filter: any record whose active interval overlaps either boundary remains visible, records without known timing leave the focused ledger, and clearing the selection restores the full branch. Wheel gestures zoom the time domain. A right-button click clears the interval selection; dragging instead pans an already zoomed viewport without mutating it. The Overview keeps the full time domain while focused so the selection can be resized or cleared without losing orientation.
-- Live history updates retain the ledger's bottom position only while the user is already following its tail. Scrolling upward clears that follow state, so streamed chunks and newly appended records do not interrupt inspection of earlier rows.
-- Token streaming reuses the finalized history inspection, layout, Request numbering, Overview projection, and search results. A frame appends only the current partial Assistant cells and searches that partial when a query is active; text and reasoning deltas do not re-fold or rescan the loaded prefix, while message completion, tool lifecycle, compaction, rewrites, and other structural events rebuild the affected projections.
-- History folding passes the loaded window's absolute starting sequence into the canonical surface manager. Structural events therefore rebuild only the entries that are present instead of materializing synthetic events for every unloaded sequence before the window.
+- Live history updates retain the ledger's bottom position only while the user is already following its tail. Scrolling upward clears that follow state, so streamed chunks and newly appended records do not interrupt inspection of earlier rows. Tail following and virtualizer measurement react to row keys and heights rather than content identity, so text-only stream frames neither discard the measurement cache nor repeat a DOM scroll write.
+- Token streaming reuses the finalized history inspection, layout, Request numbering, Overview projection, and search results. A frame appends only the current partial Assistant cells and searches that partial when a query is active; text and reasoning deltas do not re-fold or rescan the loaded prefix, while message completion, tool lifecycle, compaction, rewrites, and other structural events rebuild the affected projections. Before those rebuilds, the inspection ledger drops completed-step token payloads that no projection reads while retaining the first visible token for timing, every usage chunk for accounting, and every chunk from unfinished or interrupted steps; the independent history source retains the raw entries.
+- History folding rebases only the loaded surface events into a compact contiguous input for the canonical surface manager, then maps its nodes back to absolute session sequences. Structural events therefore retain canonical replacement validation without replaying token chunks or materializing synthetic events for unloaded sequences.
 - Trajectory opts into a conversation-owned composer overlay through `data-conversation-composer-overlay`. `ConversationRoot` positions the composer seat and publishes its live height; Trajectory keeps the ledger at full height and reserves that height plus 16 px inside its vertical table and inspector scrollers. Those panes adapt to the available width instead of exposing horizontal scrollbars beneath the overlay.
 - This local inspector remains independent from the conversation-wide Chat details column. At narrow widths it overlays the ledger and remains dismissible by keyboard or pointer.
 
@@ -53,4 +53,4 @@ Trajectory has to make prose, machine payloads, token usage, timing, and nested
 
 ## Consequences
 
-Trajectory shows more useful records per viewport while retaining Turn and Request orientation. Context rewrites and compactions remain inline with their surrounding history, while a rewind begins a successor branch that inherits only the retained prefix. The floating composer leaves the ledger visible to the viewport edge without covering its final rows or hiding horizontal controls. The main ledger omits token usage and duration so content receives the available width; the local inspector exposes those facts together with full payloads, provenance, schemas, and request timing. The Overview uses recorded start/duration and token-boundary facts without fabricating live elapsed time, and its inclusive focus behavior matches the interaction users already know from Chrome DevTools Network. Tail-first paging bounds initial transport and projection work, virtualization bounds mounted row elements, and incremental partial projection removes loaded-history length from ordinary token-frame work; structural rebuilds remain linear in the loaded window rather than its absolute tail sequence. Focused component tests pin tail-first paging, prepend anchoring and identity retention, the virtual window, tail following, streaming structural sharing, high-sequence window folding, timing projection, delayed detail disclosure, folding, record and interval selection, entity-specific tabs, and running/error semantics; the assembled Web snapshot pins the ledger, Overview timing details, composer overlay geometry, and inspector through the real client composition.
+Trajectory shows more useful records per viewport while retaining Turn and Request orientation. Context rewrites and compactions remain inline with their surrounding history, while a rewind begins a successor branch that inherits only the retained prefix. The floating composer leaves the ledger visible to the viewport edge without covering its final rows or hiding horizontal controls. The main ledger omits token usage and duration so content receives the available width; the local inspector exposes those facts together with full payloads, provenance, schemas, and request timing. The Overview uses recorded start/duration and token-boundary facts without fabricating live elapsed time, and its inclusive focus behavior matches the interaction users already know from Chrome DevTools Network. Tail-first paging bounds initial transport and projection work, virtualization bounds mounted row elements, incremental partial projection removes loaded-history length from ordinary token-frame work, and completed-step chunk compaction makes structural rebuilds proportional to inspection-relevant entries rather than the raw token count. Focused component tests pin tail-first paging, prepend anchoring and identity retention, the virtual window, tail following, content-only streaming without repeated scroll writes, streaming structural sharing, high-sequence window folding, timing projection, delayed detail disclosure, folding, record and interval selection, entity-specific tabs, and running/error semantics. A real-browser long-ledger contract pins stable prepend geometry, bounded mounting, top/middle/bottom reachability, and bounded scroll writes across a paced stream; the assembled Web snapshot pins the ledger, Overview timing details, composer overlay geometry, and inspector through the real client composition.