[foxnne]

Part of plans/RECORDING_PLAN.md (fizzyedit/plans#17), step 5, first half: the transport. The hot zone and pixi's history line follow.

What changes

Undo and redo show a document's transport (app.HistoryTransport): a bar of glass at the bottom centre of that document's own pane.

• What's on it: its history as a line with a tick per step (while they stand 6 points apart) and a knob where the document is; undo and redo bubbles either side; the step's title with how far along it is ("Edit 2 / 3").
• The look: three bubbles of one liquid glass (core.dialogs.glassBubbles), as the demo player's transport is.
• The line: a press on the line's band goes to the step under it, and a drag follows the pointer. That's at most one historyGoTo a frame, so a drag across a thousand steps costs a jump a frame. A press on the title does nothing. A drag lasts only while the bar holds the mouse: if the bar stopped drawing mid-drag (a claim came), the drag is over. The bubbles step one at a time.
• Coming and going: the bar fades in, stays while the pointer is on it or a drag holds it, and goes 3 s after it was last touched. While it shows, it sets one timer for its next change (fully in, starting to go, gone); once gone it sets none and asks nothing of the owner.
• A held undo or redo goes further each press: one step a press for the first six, then 2, 4, 8, … up to 32. A press the other way starts a new run. Each step is the owner's own undo/redo command, so anything an owner does around an undo still happens. The bar doesn't show on a claimed document, or when there's nothing to undo or redo.
• The activity feed stays above the bar, as it does above a claim's pill. The bar's place is computed before either draws (place), so they don't overlap even on its first frame. A first version used the bar's rect from the frame before, and they overlapped on the frame it came in.
• Anchors history.line:<path> (its x is that fraction of the history), history.back:<path> and history.forward:<path>, unprefixed like app/'s others (region:, picker:), for tapes and tests. Documented in docs/AUTOMATION.md.

For any owner with historyLine and historyGoTo, so text has it now. Pixi needs both (it has step ids since fizzyedit/pixi#45). That's in the next half, with the hot zone: the pointer near a view's bottom centre, no button held, a short dwell, bringing the bar up.

The rules are std-only and tested on their own (app/history_transport.zig, fizzy-history-transport-tests): when the bar shows and for how long, how far a held key goes, and which step a point on the line is.

Not built yet:

• Going back then editing still clears redo, as undo always has. The timeline plan's "going back is a step" (HISTORY_TIMELINE_PLAN.md step 4) is what keeps it on the line.
• Authors and times on the line. Authors come with SHARED_HISTORY_PLAN's step 2; times are in the tracks already.

SDK impact

• None
• Core-only or additive: reaches plugins at the next SDK release
• Fingerprint moved: recorded in sdk/src/version.zig, sdk_version untouched, PR labelled sdk
• SDK release: bumps sdk_version, lists the sdk PRs since the last sdk-v* tag

Verified

• macOS:
  • scripts/check-fmt.sh, zig build, zig build check-web, zig build test-sdk-version and zig build check-tests-reached pass.
  • zig build test: 572/573 (1 skipped), including the transport rules. zig build test-integration: 53/53 steps, 542/542 tests.
  • The new integration test makes 40 steps in a text document and undoes. It checks:
    • the bar shows, at 39;
    • a live tape clicking the title over the line's start leaves it at 39;
    • clicking the line's start goes back before every change (0), and clicking redo goes to 1;
    • a drag along the whole line ends at 40;
    • ten undos in one frame go 14 steps, landing at 26.
  • Seen in a running fizzy (Debug, driven through the agent plugin's bridge): the bar over the document undo was pressed in, its label, ticks and knob, and the feed's bubbles above it once its place was computed first.
• Windows: not run.
• Linux: CI.
• Web: check-web builds. Not seen there yet.

Follow-ups

• The hot zone, and historyLine / historyGoTo for pixi (the second half of step 5).
• "Going back is a step", so scrubbing back and editing loses nothing.
• The line is asked for each frame the bar shows. HistoryLine.version can't save that, because reading the version means building the line. A historyVersion hook (one number, no allocation) is the seam, for when long histories make it show up in the profiler.

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