A drop shadow that dims what it covers

[ralyodio]

The last of #64. A shadow cast by a region onto whatever is behind it, and the point of it is that it dims rather than paints: a shadow that filled its band with a flat colour would erase the dashboard underneath, which is the opposite of what a shadow is for. Every covered cell keeps its character and its hue and only loses some of its light.

ui.ctx.overlay(root => {
  drawShadow(root, dialogRect);
  // ... then draw the dialog into the same rect
});

dimRect is the effect on its own, for anyone who wants it without the geometry. A color option paints instead, for a shadow falling on empty background where there is nothing to dim.

Two bugs the ports found

The colour encoding has a sentinel. Zero is not black — it means "the terminal's own colour" — so the first version mixed towards it and drained every cell it touched instead of darkening it. Rust disagreeing with the fixture is what surfaced it; the TypeScript looked perfectly plausible on its own. It is rgb(0, 0, 0) now, in every port, and the fixture that captured the wrong behaviour was regenerated.

The corner was dimmed twice. Splitting the shadow into "the band down the side" and "the band along the bottom" overlaps where they meet, and a cell dimmed twice is visibly darker than the rest — it reads as a smudge rather than an edge. The shadow is now the moved region minus the original, cut into two rectangles that do not touch. A test counts the shades.

One decision

Dimming goes towards black rather than towards the theme's background. On a light theme the background is the light, so dimming towards it would make the shadow brighter than the page it falls on.

Verified

• 339 TypeScript tests under both bun test and node --test, typecheck clean
• 90 widget scenes matching the reference in Rust, Go, Python, Zig and C++ (was 86)
• 11/11 ctest, the widget galleries, next build for the site

Six ports, four fixtures, all additive. Closes #64.

🤖 Generated with Claude Code

https://claude.ai/code/session_017Df2FNu5DhinMV2soRz3cy