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. Work that is finished lives in "the architecture, as
built"; the running log of how each piece got there has been dropped. `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, but its `Markdown()` composable is not called at all:
`MarkdownRoot` in `Markdown.kt` provides the `Local*` environment itself --
reference links from the parse, padding, dimens, colours, typography, a
no-op image transformer, animations, components -- and `MarkdownElement`
dispatches a whole block through our component table. Nothing between a
piece and the screen is the library's now except the leaf composables that
table names.

**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 would reparse the whole list per delta, since it is one
tail block; that is what the rule below cuts.

**A streaming list becomes a unit per item.** `LiveParse.advanceTo` cuts at
the last item of a multi-item list (`openPiece`), provided that item has
content beyond its marker -- a bare `-` is an empty item now and the first
character of a paragraph line once `-x` arrives, so cutting on it would draw
that line as a new item. The cut is at the start of the item's line, so the
indentation the reparse reads its nesting from survives. `Segment.continues`
marks a tail that carries on a list, and `MarkdownPiece`'s
`continuesList`/`listContinues` keep an inner item's padding at the seam, so
nothing moves when the seam does. Forty linked bullets streamed a word at a
time went from 2412ms of reparsing to 674ms, and `record: one block` from
1.8ms worst to 0.7ms.

**Fences are highlighted off the drawing thread, and a fence still being
written is drawn plain.** `CodeFence.kt` holds `highlight` (shared with a
tool call's input, so the same code is the same colours wherever it
appears), the `fenceLanguage` alias table, and `fenceContent`. A word not in
the table stays plain, because a fence coloured by the wrong language's
rules looks highlighted and is wrong in a way the reader cannot see.
Highlighting is warmed and cached exactly as parsing is
(`ParsedReplies.highlighted`, filled by `warm` from `fences(parse)`), and
`highlight` takes no colour from the theme, which is what lets it run off
the drawing thread: a two-hundred-line Kotlin fence costs 174ms to lex, and
a `remember` inside the fence was charged that again every time the block
scrolled back into composition. Because the warming has to ask for the same
string the drawing does, `fenceContent` extracts the code and the language
word itself -- two extractions would be two keys, and the warmed answer
would be missed at every fence with nothing saying so. A fence still
arriving is the same stall in a second place, and warming cannot reach it:
the tail was re-lexed at every delta, on the composing thread, for colours
on text being replaced as fast as they were computed -- 211 lexes and 13.7
seconds across one turn. So `MarkdownRoot`'s `streaming`, true only for a
live reply's last segment, draws the block plain until it freezes; a
finished fence colours as soon as the next block starts, and the settling
lex happens once, in `warm`.

**Markers, and images.** `MarkdownListItem`'s `Marker` draws the bullet by
depth, cycling past the third, in `listMarkerColor` (Theme.kt). The colour
is the same at every depth on purpose: depth is said by the glyph and the
indent, and a colour per depth would make a difference in degree look like
one in kind. The app has no image loader and the renderer's transformer was
the no-op one, so an image in a reply drew as *nothing at all*; an `IMAGE`
node is now appended by `appendPlainLink` as a link carrying its alt text
(the address when there is none), which says what was there and opens it.

**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.
- **`app/stream-bench.sh [-k] FILE`** is `transcript-bench.sh` for a reply
  still arriving: opens the first session, taps "Jump to latest" so the list
  is pinned to the newest end, resets the report, sends FILE, waits for the
  transcript to stop growing, prints the report. Both of those last two are
  corrections to a first version that measured nothing -- a transcript parked
  further back never redraws while a reply streams into it, and a session is
  idle at *both* ends of a turn, so polling for idle answers before the turn
  has started. Fixtures live in `/tmp` and are regenerated from the shapes
  named here: `fixture.md` (lists four deep, ordered and nested, fences in
  kotlin/rust/sh/none, a table with a link, a quote with a list, an inline
  and a standalone image, a reference link), `longfence.md` (200-line Kotlin
  fence), `longlist.md` (40 linked items).
- **Two traps in the emulator loop**, each of which cost a bench run.
  `adb shell pm clear` removes the enrolment and the notification permission
  along with the saved anchors, so the next run measures a permission
  dialog; re-enrol with the command `ui-sandbox.sh` prints and
  `pm grant ... POST_NOTIFICATIONS`. And a saved anchor is per session id,
  so the only way two builds start a scroll from the same place is a *fresh
  session for each*.
- **`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 finds comments before it knows the language, and pairs
  `/*` with `*/` by ordinal.** Measured against the library directly with
  `app/highlights-repro.sh`, which asks it for a piece of code outside the
  app and prints every span with the text under it -- the mistakes are
  invisible on a phone, where a line greyed out as a comment looks like a
  comment. In `MultilineCommentLocator` and `CommentLocator`: it collects every `/*` and
  every `*/` in the code, zips the two lists by position and never checks
  that the end follows the start, so `x '*/a/*'` yields `start=6, end=5` --
  a range `AnnotatedString` rejects, which crashed a card holding
  `-path '*/.git/*'` until `highlight` started dropping such spans. The same
  delimiters are used for every language, so a shell glob is read as a
  comment opener and, worse, `//` in any URL comments out the rest of its
  line: in `curl https://example.com/x && echo done` the comment runs to the
  end and takes `echo` with it, and in Kotlin `val url = "https://..."` the
  string span disappears inside it. Comments are located before strings and
  win over them. What we can reach is `getCodeStructure()`, which is public,
  plus a nine-line reconstruction of the library's private
  `constructHighlights`; the locators themselves are `internal`. 1.1.0 is
  the newest release, so there is nothing to upgrade to.

## 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. **The reconnect loop.** Restarting the app onto a session with a saved
   anchor while a long reply was streaming left it reconnecting every 1.5s
   (`RECONNECT_DELAY_MS`), spinner up, until the server was restarted.
   `events?after=N` more than `CATCH_UP_LIMIT` (200) behind answers `reset`
   plus the newest 200 *raw* deltas -- a window starting mid-message -- and
   the reset clears `items`, which is the state the restore loop then pages
   against. The restore's one-event-per-request bug was part of what made it
   so visible and has been fixed; whether this survives that fix is the
   first thing to find out.
2. **Decide what to do about the highlighting bug above.** Two options, both
   ours: post-process `getCodeStructure()` -- pair `/*` with the next `*/`
   after it, and only for languages that have them, ignore `#` and `//`
   where the language does not use them -- then build the annotated string
   from the corrected structure, which costs no extra lexing but cannot
   recover keywords and strings the wrong comment range already suppressed;
   or fork the twenty-file library and fix the locators. Filing it upstream
   needs Bryan or a token, since there is no `gh` and no GitHub credential
   in this VM. One-line repro: lexing `x '*/a/*'` as `SyntaxLanguage.SHELL`
   in highlights 1.1.0 returns a highlight whose `location.end` precedes its
   `location.start`. Whichever is chosen, `app/highlights-repro.sh` is what
   checks it.
3. **Regression runs.** `transcript-bench.sh` and `stream-bench.sh` before
   and after any change to the files above, with the report in the commit.
   The numbers to watch are the worst `record: one block`, the reparse mean
   while streaming, and the draw phase's accounting line.