feat: name the region that would not settle, and document the readiness recipes (#271, #273) - #280
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📝 WalkthroughWalkthroughThe PR adds detailed reports for unstable screenshot regions and run-level metrics for captures that require stabilization. It adds readiness guidance for webfonts and lazy-loaded images, plus unit, parallel, and browser integration tests. ChangesScreenshot stability diagnostics
Estimated code review effort: 3 (Moderate) | ~25 minutes Merge Risk: 🟡 Moderate · up to The change improves instability diagnostics, but overlapping changed regions can currently be reported as separate areas, leading to incomplete masking guidance and unreliable stabilization troubleshooting. This should be fixed before merge; the test fixture should also use standards mode for stable measurements. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Reviewer's GuideThe PR turns stability failures from opaque attempt-path dumps into measured, actionable diagnoses, adds fork-safe run-level evidence for successful settling costs, and documents readiness-block recipes for fonts and lazy-loaded content. Sequence diagram for measured screenshot stabilizationsequenceDiagram
participant StableScreenshoter
participant Snapshot
participant Comparison
participant AttemptsReporter
participant Reporting
StableScreenshoter->>Snapshot: attempt_next_screenshot(snapshot)
loop Until stable or timeout
StableScreenshoter->>Comparison: attempt_successful?(snapshot)
alt Screenshot is stable
StableScreenshoter->>Reporting: record_stable_capture(seconds, attempts)
else Screenshot changed
StableScreenshoter->>StableScreenshoter: sleep(stability_time_limit)
StableScreenshoter->>Snapshot: attempt_next_screenshot(snapshot)
end
end
StableScreenshoter->>AttemptsReporter: generate()
AttemptsReporter->>Comparison: build_comparison_for(attempt_path, previous_attempt_path)
Comparison-->>AttemptsReporter: difference.region
AttemptsReporter->>AttemptsReporter: cluster(regions)
AttemptsReporter-->>StableScreenshoter: diagnosis with measured skip_area or readiness advice
Flow diagram for actionable stability failure diagnosisflowchart TD
A[Compare consecutive screenshot attempts] --> B{Changed regions present?}
B -->|No| C[Report stable attempt]
B -->|Yes| D["cluster(regions)"]
D --> E{Area changed in every attempt pair?}
E -->|Yes| F[Report measured skip_area]
E -->|No| G[Report rendering churn]
F --> H[Use readiness block or skip_area]
G --> I[Settle page in readiness block or raise wait]
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Inline comments:
In `@lib/snap_diff/attempts_reporter.rb`:
- Around line 105-113: Update the regions aggregation logic around Area so each
incoming region is merged with every transitively overlapping area, combining
all intersecting areas into one and accumulating their pair counts plus the new
region. Add a regression test covering two separate areas followed by a bridging
region, and verify they produce a single merged area suitable for
animating_lines to suggest skip_area.
In `@test/fixtures/app/index-with-ticker.html`:
- Line 1: Add the HTML5 standards-mode doctype before the root html element in
the ticker fixture, preserving the existing document content and structure.
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docs/configuration.mdlib/snap_diff/attempts_reporter.rblib/snap_diff/reporters/default.rblib/snap_diff/reporting.rblib/snap_diff/stable_screenshoter.rbtest/fixtures/app/index-with-ticker.htmltest/integration/browser_screenshot_test.rbtest/support/test_doubles.rbtest/unit/attempts_reporter_test.rbtest/unit/parallel_report_merge_test.rbtest/unit/reporting_counts_test.rbtest/unit/stable_screenshoter_test.rb
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…a doctype Both from CodeRabbit review on #280. The clustering merged an incoming region into the FIRST area it touched. A chain -- A touches B, B touches C, A does not touch C -- therefore left two areas instead of one. The lane's own comment argued that under-counting is safe because it cannot invent a mask, and that is true, but it misses the cost: each fragment is then seen in fewer attempt pairs than the whole, so a single animation is classified as churn and NO mask is offered. It withholds the one suggestion this message exists to make. Now merges every touching area and sums their pair counts. Guarded with the bridge case; the guard reds under first-overlap-wins. The ticker fixture had no doctype, so browsers rendered it in quirks mode -- different box model, in a fixture whose entire purpose is pixel comparison.
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…a doctype Both from CodeRabbit review on #280. The clustering merged an incoming region into the FIRST area it touched. A chain -- A touches B, B touches C, A does not touch C -- therefore left two areas instead of one. The lane's own comment argued that under-counting is safe because it cannot invent a mask, and that is true, but it misses the cost: each fragment is then seen in fewer attempt pairs than the whole, so a single animation is classified as churn and NO mask is offered. It withholds the one suggestion this message exists to make. Now merges every touching area and sums their pair counts. Guarded with the bridge case; the guard reds under first-overlap-wins. The ticker fixture had no doctype, so browsers rendered it in quirks mode -- different box model, in a fixture whose entire purpose is pixel comparison.
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Author
Full matrix: 2 runs, 2 failures, different cells each time
1. Fixed here — a one-pixel change suggested a mask covering nothing (
|
| run | suggested mask |
|---|---|
3.4/rails81 |
[216,52,216,65] (1 column) |
4.0/rails71 |
[62,50,219,67] (157×17) |
A later frame can render outside the observed box, and the masked run is unstable again. The
message's promise — "Exclude it and the page is stable without waiting" — is absolute,
but for a variable-extent animation it is only probabilistic.
This needs a call I did not want to make on your behalf under release pressure:
- pad the suggested mask outward — principled (the observed box is a sample of a moving
thing) but the padding is a magic number that can still under-cover; - make the fixture fixed-extent (monospace) — deterministic, but the fixture's comment says
the every-character churn is deliberate, so this risks rewriting the evidence rather than
fixing the flaw; - soften the message's promise for clusters whose extent varies across pairs.
Scheduling
Dropped from 2.0.0.beta4 so a flaky test does not land on a master we just spent the
day getting green. Nothing here is lost — 606a0b9 stays on the branch, and #271/#273 are
worth shipping as soon as (2) is settled.
Collaborator
Author
Both blockers resolved; rebased onto
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…a doctype Both from CodeRabbit review on #280. The clustering merged an incoming region into the FIRST area it touched. A chain -- A touches B, B touches C, A does not touch C -- therefore left two areas instead of one. The lane's own comment argued that under-counting is safe because it cannot invent a mask, and that is true, but it misses the cost: each fragment is then seen in fewer attempt pairs than the whole, so a single animation is classified as churn and NO mask is offered. It withholds the one suggestion this message exists to make. Now merges every touching area and sums their pair counts. Guarded with the bridge case; the guard reds under first-overlap-wins. The ticker fixture had no doctype, so browsers rendered it in quirks mode -- different box model, in a fixture whose entire purpose is pixel comparison.
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…281) (#292) The matrix stays off PRs for free-tier minutes, and the consequence is that the job which breaks master is a job that never ran on the PR. Three breakages this week, all invisible on a green PR for exactly that reason: - a Rails 7.1-only constant (#283) - JRuby not implementing Kernel#fork (#283) - a zero-width skip_area mask (#280) Every one was found by adding `full-ci` BY HAND after master had already gone red, which is a process that works only when someone remembers. All three came from `test/`, `gemfiles/` or `.github/`, so the matrix now runs automatically when a PR touches those. The label stays for everything else. Paths are asked for over the API rather than `git diff`: checkout is depth-1, so the base commit is not in the clone to diff against. `lib/` is deliberately NOT on the list. It changes on nearly every PR, and the functional and minimal-setup jobs already cover it -- putting it here would run 25 cells on almost everything and give back the cost decision the exclusion exists to make. That is a trade, not a claim that `lib/` is safe, and CONTRIBUTING.md says so. CONTRIBUTING.md also gets the two reading rules that cost real time this week: `cancelled` is not a pass, and `gh run list --branch master` without `--workflow Test` will hand you whichever workflow ran last -- it reported a Dependabot success while `Test` was failing on the same commit. Detection self-tested against real paths; YAML validated.
#271) Stabilisation failures printed a bare list of attempt paths. Diagnosing one meant opening N PNGs and eyeballing them -- so `sleep 2` won, and suites got SLOWER as a consequence of diagnosis being hard. A maintainer reported a 10-minute suite dominated by stabilisation waiting, with sleeps adopted deliberately "to avoid debugging as much as possible". The information was already here and thrown away: AttemptsReporter compares every consecutive pair of attempts, and that comparison knows the region that changed. Print it, with the escape hatch: Could not get stable screenshot for 'index-with-ticker' within 1.2s (5 attempts). The page kept changing in 1 area, over 4 attempt pairs: [67,50,213,68] (left,top,right,bottom edges) -- 0.55% of the 800x600 image, changed in 4 of 4 pairs Always the same area, in every pair: that is an animation, clock, carousel or live counter. Exclude it and the page is stable without waiting: assert_matches_screenshot "index-with-ticker", skip_area: [67,50,213,68] <attempt paths> Animation vs churn is decided by count, not by shape: regions are clustered by overlap, and a cluster present in EVERY attempt pair is animating -- skip_area fixes it. Anything less is the page still rendering, where masking would hide real content, so the message says so and suggests nothing to mask. The suggested coordinates are the ones just measured. Guarded by following the advice on a real browser and a really unstable page (test/fixtures/app/ index-with-ticker.html): the failing run's own suggestion, pasted back in, makes the page stable. Fabricating the coordinate reds that test -- which is the check this gem lacked when it shipped RECORD_SCREENSHOTS=1 in its own error message for years while nothing read it. Success path: the run-level summary now reports the worst stabilisation it saw. A user who set `stability_time_limit: 2` had no way to learn their pages settle on the first retry, and without evidence tuning it down is guesswork. Run-level rather than per-assertion (per-test noise is the last thing a slow suite needs) and silent when nothing waited -- the same rule as the never-matched-selector line. It rides the fork-parallel fragment, since a run-level line that vanishes under Rails' default parallelize is #269 again; counts add, worst cases max. Pairs with #272 (masking is instant) and #279 (dead selectors are surfaced): "here is the region, mask it" is finally a complete workflow.
#279 shipped the optional block on assert_matches_screenshot / capture_screenshot, but documented only that it exists. Nobody finds a mechanism without the use-cases, and the use-cases here are exactly the workarounds real users already hand-roll. Two recipes, both in docs/configuration.md next to the block and cross-linked from the skip_area section: - Webfonts. A font swapping mid-capture reflows text bimodally -- the "only fails on CI" flake people paper over with a skip_area, a loosened tolerance and a retry, all three of which weaken the comparison everywhere. `document.fonts.ready` waits for exactly the swap and returns on the first round trip once fonts are cached. - Lazy images. Scroll, wait for something at the bottom, scroll back -- and note the ORDER: skip_area masks what exists at assertion time, so a selector for content that has not loaded yet produces an empty mask and the unstable region is compared anyway. Plus what does NOT belong in the block, and why there is no built-in font wait: it would be a browser round trip imposed on every screenshot in every suite, and a driver-compatibility surface the gem would own forever, in exchange for one line a user can write.
CI produced [62.0,50.0,218.0,68.0] where macOS produced integers, and the message's own regex (`skip_area: (\[[\d,]+\])`) silently failed to match -- so the integration test that pastes the suggestion back in could not find it. Two defects, not one. Float coordinates are not pasteable into a test file. And the naive fix, truncation, would shave the right and bottom edges and leave the moving pixels exposed -- a mask that under-covers is worse than no suggestion, because it looks like it worked. Round OUTWARD: floor the near edges, ceil the far ones. Guarded, and the guard reds under truncation.
…a doctype Both from CodeRabbit review on #280. The clustering merged an incoming region into the FIRST area it touched. A chain -- A touches B, B touches C, A does not touch C -- therefore left two areas instead of one. The lane's own comment argued that under-counting is safe because it cannot invent a mask, and that is true, but it misses the cost: each fragment is then seen in fewer attempt pairs than the whole, so a single animation is classified as churn and NO mask is offered. It withholds the one suggestion this message exists to make. Now merges every touching area and sums their pair counts. Guarded with the bridge case; the guard reds under first-overlap-wins. The ticker fixture had no doctype, so browsers rendered it in quirks mode -- different box model, in a fixture whose entire purpose is pixel comparison.
CI (3.4/rails81) failed the integration test that pastes the suggestion back
in, because the suggestion was degenerate:
The page kept changing in 1 area, over 2 attempt pairs:
[216,52,216,65] -- <0.01% of the 800x600 image, changed in 2 of 2 pairs
Exclude it and the page is stable without waiting:
assert_matches_screenshot "index-with-ticker", skip_area: [216,52,216,65]
left == right. Region carries WIDTH and `from_edge_coordinates` derives it as
`right - left`, so that mask is 0 px wide: it masks nothing, the page still
does not settle, and the user is told to paste a fix that cannot work.
A ticker digit or a caret is one column wide, which is exactly when the two
edges collapse -- so the message was worst precisely where it was most needed.
Timing-dependent, which is why it passed locally and on 15 other cells.
This is the degenerate case of the invariant the outward rounding already
states -- "a mask that under-covers is worse than no suggestion, because it
looks like it worked" -- taken to the limit where it covers nothing at all.
floor/ceil cannot reach it: the edges are already integral.
Floor the near edges as before, then require at least one pixel of extent on
each axis. Reproduced first as a deterministic unit test (the CI failure needs
a real browser and the right millisecond); mutation-checked by reverting to
plain ceil, which reds it.
standardrb clean, `rake test` 771 runs / 2289 assertions / 0 failures.
… sample
The round-trip test -- paste the message's own suggestion back in, page must
then be stable -- failed twice on CI, on two different cells (runs 32750597989
and 32752142873), with well-formed regions both times:
[216,52,216,65] 3.4/rails81 (zero-width; fixed in 606a0b9)
[62,50,219,67] 4.0/rails71 157x17, and the masked re-run STILL failed
The second is not a bug in the measurement. `index-with-ticker.html` re-randomises
all ten characters every 30ms inside a `text-align: center` box, and in a
PROPORTIONAL font ten random glyphs render to a different WIDTH each tick. The
suggested box is the union of what changed across the attempts that ran -- a
sample of a moving target -- so a later frame can render outside it.
Two changes, because there are two separate facts here.
**The fixture** pins the font to monospace. The pixels still change completely
every tick, which is the property under test; only the extent stops moving, which
is not. That makes the round trip deterministic instead of a coin flip on how
many attempts happened to sample.
**The docs** state the limitation rather than hide it, because it is real for
users too: for an animation whose SIZE varies frame to frame the first suggestion
can under-cover, the failure then reports a much smaller region, and pasting the
new one converges. For the usual case -- a clock or spinner repainting inside a
fixed element -- the extent does not move and the first suggestion is the fix.
Deliberately NOT done: padding the suggested box by a fixed margin. The number
would be arbitrary, it can still under-cover, and it would widen every correct
suggestion to paper over a case the message can simply be honest about.
`rake test` and standardrb below.
The round trip -- fail, parse the message's own suggestion, apply it, page must
then be stable -- failed on three separate CI runs with three unrelated regions
for the same page:
[216,52,216,65] 1 column
[62,50,219,67] 157x17
[71,51,71,68] 1 column
The diagnosis was right every time. What it was diagnosing would not hold still.
Ten random glyphs are a bad thing to measure. Their extent depends on which
characters came up, and a capture on a slower machine can land mid-repaint and
see a single column of a single character -- hence regions ranging over two
orders of magnitude. Pinning the font to monospace (previous commit) fixed the
extent but not the mid-repaint sliver, so it was necessary and not sufficient.
Now every tick paints the box a RANDOM colour. Two attempts then differ across
the whole element at high contrast, a partial repaint is still an unmistakable
diff, and the region is the element -- which `position: absolute` with a fixed
width and height pins exactly.
Random, specifically, and not a black/white toggle: a two-state flip depends on
parity, and two attempts ~100ms apart are an unpredictable number of 30ms ticks
apart, so they can land on the SAME phase. Measured -- with the toggle the region
came back as [69,50,210,66], the text again.
Measured after: eight consecutive local runs, all green, every one reporting
[40,40,239,79] -- 1.62% of the 800x600 image, changed in 4 of 4 pairs
which is the CSS box (left:40 top:40 200x40) to the pixel. Before this change no
two runs agreed.
This is also the honest shape of what the fixture stands in for: a clock or
spinner repainting inside a box that does not move, which is exactly the case
where `skip_area` is the right answer.
`rake test` and standardrb below.
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Closes the two loops v2 opened.
1. Name the region that would not settle (#271)
Stabilisation failures printed a bare list of attempt paths. Diagnosing one meant opening N PNGs and eyeballing them — so
sleep 2wins, and the suite gets slower as a consequence of diagnosis being hard. A maintainer reported a 10-minute suite dominated by stabilisation waiting, with sleeps adopted deliberately "to avoid debugging as much as possible".The information was already here and thrown away:
AttemptsReportercompares every consecutive pair of attempts, and that comparison knows the changed region.Before
After (real output, real browser,
test/fixtures/app/index-with-ticker.html)Churn reads differently and suggests nothing to mask:
Animation vs churn
Decided by count, not shape. Per-pair regions are clustered by overlap; a cluster present in every attempt pair is animating and gets a
skip_area. Anything less is the page still rendering, where masking would hide real content. Mixed pages get both blocks — the steady areas are offered for masking, the rest named as churn.The suggestion is the measurement
The
skip_areaprinted is the region that comparison just measured. Guarded by following the advice: the integration test parses the coordinates out of the failure message and re-asserts with them, on a real browser and a really unstable page. Replacing the coordinate with a plausible constant reds that test — which is the check this gem lacked when it shippedRECORD_SCREENSHOTS=1in its own error message for years while nothing read it.Success path
The run-level summary now reports what waiting actually cost:
Run-level rather than per-assertion or
DEBUG-gated: per-test noise is the last thing a slow suite needs,DEBUGis a flag nobody sets before they already suspect a problem, and the number is only actionable as a run-wide worst case anyway. Silent when nothing waited — same rule as the never-matched-selector line. It rides the fork-parallel fragment, because a run-level line that vanishes under Rails' defaultparallelizeis #269 all over again; counts add up, worst cases max.Pairs with #272 (masking is instant) and #279 (dead selectors surfaced): "here is the region, mask it" is finally a complete workflow rather than advice with a 5-second tax.
2. Document the readiness recipes (#273)
docs/configuration.md: the webfont and lazy-image recipes next to the block, cross-linked from theskip_areasection. Covers that readiness work belongs in the block because it is skipped when screenshots are off, thatskip_areamasks what exists at assertion time so late-loading targets must settle first, and thatdocument.fonts.readyreturns on the first round trip once fonts are cached.No fonts feature: a built-in wait would be a browser round trip imposed on every screenshot in every suite and a driver-compatibility surface owned forever, for one line a user can write.
Verification
rake test,rake test:canonical,rake test:unit,standardrb lib test— green locally and underCI=true.Six mutations run; every guard confirmed red on break and byte-identical on restore. One mutation survived and was a finding: the fork-parallel test recorded the worst case in a worker, so a last-write-wins merge passed by pid-sort luck. The fixture now holds the worst case in the parent, which is order-independent; the mutation reds it.
🤖 Generated with Claude Code
https://claude.ai/code/session_014BQJX6eWzBj2UTm5zQsjEs
Summary by Sourcery
Improve screenshot stabilization diagnostics and documentation so users can identify persistent animations, apply verified masks, and tune waiting based on observed costs.
New Features:
skip_areasuggestions for consistently animated areas.Bug Fixes:
Enhancements:
Documentation:
Tests:
Summary by CodeRabbit
New Features
skip_areacoordinates for continuously changing regions.Documentation
Bug Fixes