feat(reader): page gap for dual-page spreads + ctrl+shift+scroll (#234)

[Gnathonic]

Resolves #234.

What

• New `pagedGap` setting (0–100): renders as `column-gap` between the two pages of a paged-mode spread. The gap lives in image-pixel space, so it scales with zoom like a physical gutter. Slider appears in reader settings when page view mode is dual/auto.
• Fit math stays exact: new pure `spreadContentSize()` helper includes the gap in the paged content size, so fit-to-screen/width, pan clamping, and swipe edge detection account for it.
• Ctrl+shift+scroll live-adjusts the gap on all three reader surfaces: paged writes `pagedGap`; continuous modes write `scrollGap` and auto-sync `pageDividers` (0 = off). Keyed toast shows the current value.

Binding rationale

Wheel combos with a native browser meaning keep that meaning tuned for the reader (ctrl+wheel = zoom, shift+wheel = horizontal pan); gap adjustment takes ctrl+shift+wheel, which no browser binds. Recorded in `docs/INPUT-CONTRACTS.md`. Handles the browser quirk where shift moves the wheel delta into `deltaX`.

Verification

• 947 unit tests pass (new: `spreadContentSize`, `gapWheelSteps`/`wheelIntentIsGapAdjust`, `pagedGap` migration); svelte-check and lint clean.
• Playwright end-to-end against the dev build: imported a synthetic volume, verified flush baseline, toast + visual gap at 25px/100px (scaled by fit exactly), clamping at both bounds, ctrl+wheel zoom and shift+wheel pan unaffected, continuous-mode chord enabling dividers from zero, and per-mode setting independence.

🤖 Generated with Claude Code

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
mokuro-reader	Ready	Preview, Comment	Jul 6, 2026 5:04am