History diff

Per-pixel bounding boxes of regions that changed between two history entries (e.g. base render vs. PR head, or before / after a code edit).

At a glance


Kind	`history/diff/regions`
Schema version	1
Modules	`:data-history-core` (published) · `:data-history-connector`
Render mode	default
Cost	low
Token usage	Inline JSON, not yet benchmarked. See token usage.
Transport	inline
Platforms	Android · Desktop · shared

What it answers

Where in the frame did the pixels change between two renders of the same previewId?
How many distinct change regions are there, and how big is each (in pixels and percentage)?
Is a “tiny visual change” PR actually tiny, or did the author break a screen further down the layout?

What it does NOT answer

It does not tell you why a region changed (theme tweak vs. layout regression vs. anti-aliasing) — pair it with compose/theme or layout/inspector for that.
It does not produce a perceptual / SSIM diff — it operates on raw pixel inequality, so font hinting differences and platform GPU drift can show up as regions.

Use cases

PR comments: post a single image with the changed regions outlined, plus a one-line “12 regions, ~3.4% of pixels”.
Refactor verification: prove a state-hoisting refactor produced an empty diff against compose-preview/main.
Bisect: walk backwards through baselines to find the commit that changed pixels in a particular bounding box.

Payload shape

HistoryDiffPayload, HistoryDiffRegion in :data-history-core.

// history/diff/regions
{
  "baselineHistoryId": "compose-preview/main@7a1c…",
  "totalPixelsChanged": 9184,
  "changedFraction": 0.0085,
  "regions": [
    { "bounds": "48,200,144,232",
      "pixelCount": 412,
      "avgDelta": { "r": 18.2, "g": 4.1, "b": 2.0, "a": 0.0 } }
  ]
}

Enabling

The producer attaches when the History extension is publicly enabled and data/fetch?kind=history/diff/regions is called with a left and right entry id. The companion CI workflow is preview-comment, which posts before / after PR comments using the same machinery.

Companion products

Layout inspector — layout/inspector to attribute a changed region to a specific composable.
Theme — compose/theme to confirm a colour-shift region is from a token change rather than a regression.