Client gotchas

Solid

createMemo runs when created. A memo that reads a later const hits the temporal dead zone; read the prop directly or declare dependencies first.
Signal setters commit synchronously. Repeated discrete keyboard actions must apply immediately; easing from a lagging signal coalesces rapid presses.
on(dep, fn, { defer: true }) skips its initial run. Use queueMicrotask before reading a DOM class set by the same reactive update. ref={varName} assigns the element.

SVG <text> ignores CSS text-overflow; truncate in TypeScript. Markers default to markerUnits="strokeWidth", so arrowheads scale with zoomed strokes. A wide transparent stroke still receives pointer events and can serve as a hit target over a thin line.
A two-class selector beats a one-class selector; do not use !important to compensate. Gate motion behind prefers-reduced-motion. Store display preferences under kd:* in localStorage.
Setting display on a direct child of <details> overrides the browser rule that hides closed content. Re-hide it with an explicit :not([open]) selector; this reproduces only in a browser.
In a wrapping flex row, a zero-basis item can "fit" beside a 100%-wide sibling at 0px instead of wrapping. Put the banner outside the row or give the item a real basis; jsdom cannot measure it.

jsdom reports offsetParent as null and zero rectangles. It lacks scrollIntoView and Element.animate unless stubbed, and never fires animationend. Assert the DOM contract, including that an animation class was added, then verify behavior in a browser.
On plain http://<lan-ip>, navigator.clipboard is undefined. Optional-chaining only navigator.clipboard?.writeText(x) still lets a following .then(...) throw synchronously. Optional-chain the whole chain or use await in try/catch, as CopyButton and the drawer do. Confirm only after success; otherwise no-op silently.