Fix ?slide=N deep links resetting to the first slide

[PavelMakarchuk]

Summary

Loading a deck with ?slide=N (for example /slides/gettsim-2026?slide=5) showed slide 1 and removed the slide param from the URL. This affected every deck.

Cause

SlideshowViewer read the slide param in a mount effect that depended on searchParams. On the first commit, the URL-sync effect ran with currentStep still 0 and called history.replaceState without the param. Next's app router mirrors replaceState into useSearchParams, so the mount effect ran again with no param and reset the deck to step 0.

Fix

• Read the param once in a lazy useState initializer. The mount effect no longer reads searchParams.
• Add parseSlideParam() to lib/slide-url.ts. It clamps the step to the deck and falls back to 0 for a missing or malformed value (the old code produced NaN for ?slide=abc).
• URL updates still go through buildSlideUrl(), so the /slides basePath is preserved.

Tests

• New components/core/SlideshowViewer.test.tsx mirrors replaceState into useSearchParams the way Next does. 4 of its 5 tests failed on the old code; all pass now.
• New parseSlideParam unit tests in lib/slide-url.test.ts.
• Local: bun run test (140 passed), bun run lint (0 errors), bun run typecheck, bun run build all pass on the branch merged with current main.

Manual check (dev server, gettsim-2026, 37 steps)

• ?slide=5 opens at 6 / 37 and keeps the param (before: 1 / 37, param removed).
• Arrow keys, Home, and End update the counter and the URL; 10 rapid ArrowRight presses move exactly 10 steps.
• ?slide=abc opens at step 1; ?slide=999 opens at the last step.
• No console errors or hydration warnings.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated
policyengine-slides	Ready	Preview	Oct 13, 2026 7:49pm UTC