ai-app/TRANSCRIPT_RENDERING.md

Transcript rendering: what was learned, and what is next

Written 2026-09-03 at the end of a week of work on the session screen's transcript, so the next session can start from here rather than from a compacted context. AGENTS.md holds the one-paragraph conventions; this is the longer record: the measurements that drove each decision, the techniques that worked, the ones that did not, and the order to do the rest in. PLAN.md remains the design source of truth; nothing here contradicts it.

The goal, and where it stands

A reply of any length must scroll at the phone's 120Hz without a bump, and must keep doing so while the reply is still streaming in. Measured on the Pixel 9 Pro XL by Bryan, the transcript went from visible stalls at long replies and at lists of links to "I have to actually try to feel any bumps". The remaining work is finish and extensibility rather than performance.

The architecture, as built

Everything below lives under app/androidApp/src/main/kotlin/com/example/aiapp/.

Rows become units, and units are bounded. TranscriptUnits.kt turns a transcript row into the things the lazy list actually holds. An assistant reply is not one unit: it is one unit per piece of its markdown, so the list composes and draws a paragraph, a fence, a table or one bullet at a time. The reason is the draw phase: a row's display list holds every glyph of it and is re-recorded whenever drawing is invalidated, and the lazy list composes an item whole in the frame it scrolls into. The tallest single row still being drawn before this was 36,982px, twenty-five screens in one message. Long user messages are sliced the same way (UserChunk), through the shared cardPiece modifier that draws one card in lazy-list pieces.

One parse per message, addressed by piece. MarkdownPieces.kt's Piece(block, item) is an address into the message's single parse tree, not a substring: block indexes the root's children and item one LIST_ITEM of a top-level list. Cutting was originally done by re-parsing substrings, which cost a parse per piece and broke reference links defined at the foot of a message. ParsedReplies caches the parse and the piece list per text (of, piecesOf), warmed off the composing thread by TranscriptItems.warm. The parser is still intellij-markdown via the mikepenz renderer; the renderer's Markdown() composable is kept only as the provider of its Local* environment (MarkdownRoot in Markdown.kt), and MarkdownElement dispatches a whole block through our component table.

Lists are drawn an item at a time, by us. The renderer has no element for a single list item, so MarkdownListItem draws one: marker, then the item's children, nested lists recursing through MarkdownList. The marker is drawn in one place on purpose; styled bullets per depth go there.

Links are spans, not nodes. MarkdownLinks.kt. Compose turns every LinkAnnotation into a layout node (clipped, focusable, hoverable, clickable, outline recomputed from the text layout). A paragraph of eight links was nine nodes, and measured against the same paragraphs with each link replaced by plain words it cost 26.3ms worst measure against 5.2ms, 1.7x the place time. That was the bump at a reply's list of sources. LinkedText builds the annotated string with the renderer's own inline builder but answers links itself: colour, underline, a string annotation carrying the URL, and one tap detector for the whole text that asks the layout which glyph is under the finger. Hit-testing must check the glyph on either side of the returned caret, because getOffsetForPosition returns the nearest boundary; taps on the right half of a glyph otherwise open nothing. Headings need the ATX_CONTENT/SETEXT_CONTENT child, since the inline builder draws nothing for a node type it does not know (a week of blank headings). Tables go through LinkedTable/LinkedTableRow so cells get the same treatment.

Text draws on the platform directly. A paragraph without an image skips the renderer's MarkdownText, which charges every paragraph for the possibility of inline images (placement callback, derived inline-content map, semantics group, size animation). Paragraphs that contain an image still take the renderer's path.

Tables spread or scroll without subcomposition. The renderer used BoxWithConstraints to decide; LinkedTable uses fillMaxWidth().horizontalScroll().layout { } -- horizontalScroll passes minWidth through and lifts maxWidth to infinity, so the inner layout reads minWidth as the room available and takes max(minWidth, columns * cellWidth).

A streaming reply is reparsed one block at a time. LiveParse in Markdown.kt freezes every finished top-level block with its parse and reparses only the tail block per delta. Markdown's block rules make later text unable to alter an earlier block, with the single exception of a late reference definition, which is accepted. Measured on a 58-word stream of list, fence, table and quote: 47 tail reparses at 1.7ms mean. A single-list stream still reparses per delta because the whole list is one tail block.

Expansion anchors the edge that was tapped, and the list never moves under the reader except when pinned to the bottom with new content arriving. Those two rules are in ScrollAnchor.kt and TranscriptList.kt and are the reason several tempting simplifications were rejected.

Techniques and harness

• app/ui-sandbox.sh starts a second ai-server against a sandbox home with the echo driver, so nothing touches real sessions. spawn [title] makes an echo session and prints its id; send SID text or send SID @file sends into it; api /path [curl args] is an authenticated request. Restarting it regenerates the config but keeps enrolled tokens.
• The echo driver is the test rig (server/src/session/echo.rs, the list at the top of the file). /stream N, /mixed N, /table N, /tools N gap, /ask, /peer, /compact, /slow, /bash command each produce a shape the real CLI produces only when it feels like it. Build what a UI test needs into it rather than spending model turns.
• app/transcript-bench.sh is the standard measurement: restart, open the first session, scroll, print the render report. The report is what the "Copy render timings" button copies and also logs (adb logcat -d -s ai-app:I), and it includes the last crash's stack (CrashLog.kt), which is how a crash on the phone reaches a session here.
• DebugStats/FrameStats time our own phases (record: one block, measure: the app root) and count events (markdown reparsed while streaming, markdown cut into pieces). Add a counter before guessing.
• app/trace-draw.sh names what a scrolling frame spends inside the framework, via atrace text output, no trace processor needed. It is how the link-node cost was attributed.
• app/debug-transcript.sh loads a real Claude Code conversation onto the emulator; two faults were invisible on fixtures and obvious on it. Real transcripts are private: fixtures stay in /tmp, never in the repo.
• ui-trace reads the screen as text. Bounds print as x1,y1..x2,y2; unanchored -m patterns match labels, anchored ones do not. A row taller than the viewport reports clipped bounds, so compare screenshots for that case.
• Emulator frame times are not app measurements. Software rendering puts the stock Settings app at 60ms of UI-thread traversal per frame. Costs of operations in milliseconds rank correctly; smoothness itself is judged on the phone.
• System Tracing on the phone does not work on GrapheneOS. Its Categories list is empty because the tracing daemon builds it by running atrace --list_categories, which returns nothing there, and a recorded trace contains zero ftrace events: no app sections, no frames, no scheduling. Callstack sampling records, but the app's profiler config unwinds one process shard in four. GrapheneOS issues 2206 and 6094 are open on exactly this. Until they close, phone numbers come from the render report and from Bryan noticing.
• Compose DropdownMenu in an edge-to-edge activity needs PopupProperties(clippingEnabled = false) or it opens a status bar's height away from its anchor (~/.claude/TOOLCHAIN.md).
• highlights 1.1.0's shell lexer returns a span whose end precedes its start for x '*/a/*'; ToolInput.kt drops such spans. The fix belongs upstream and has not been filed.

Rejected, and why

• Writing our own markdown renderer. Rejected in favour of keeping the intellij-markdown parser and the library's inline builder while owning block dispatch and the leaf composables. The parser is the hard part and is not the slow part; everything that was slow lived in the composables, which are now ours.
• Re-parsing substrings per piece. Cost a parse per piece and broke foot-of-message reference links. Replaced by addressed pieces of one parse.
• Animated or timing-dependent corrections. Anything the reader could catch at 120Hz is a bug; corrections must be structurally impossible to see.

What is next, in order

1. Styled markers per depth. MarkdownListItem's Marker is the one place bullets and numbers are drawn; give it a glyph per depth and the list's own colour. Bryan asked whether the architecture allows it; it does, and it is a small change.
2. Syntax highlighting inside fences. The highlights lexer used for tool commands can colour code blocks too; route the fence composable through the same table as ToolInput.kt, keep the inverted-range guard, and measure a long fence before and after, since a highlighted fence is one Text with many spans.
3. Drop MarkdownRoot's dependence on the library's Markdown(). It exists only to provide LocalMarkdown*. Providing those locals directly removes the last library composable from the hot path and frees the way for a different parser later.
4. Paragraphs with images still take the renderer's MarkdownText. Draw the image as its own piece below the paragraph instead, then the text leaf covers every paragraph.
5. Per-item units for a streaming list. A single-list stream reparses the whole list per delta; freezing finished items would make a forty-item list stream like forty paragraphs.
6. Regression runs. Run transcript-bench.sh before and after any change to the files above and paste the report into the commit. The numbers to watch are the worst record: one block and the draw phase share in the accounting line.
7. Tooling debt. AGP 9.4.0 is available (lint warns). File the highlights range bug upstream with the one-line repro.