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diff --git a/ui/lib/swatch/derive.js b/ui/lib/swatch/derive.js
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+// A fun fact about $derive: if the deriving expression throws an error, then Svelte (at least as of
+// 5.20.2) _permanently_ stops evaluating it, even if the inputs change. To prevent this, you need
+// to ensure that derive expressions never raise an exception.
+function tryDerive(args, func, fallback) {
+  try {
+    return func(...args);
+  } catch (e) {
+    console.debug('deriver threw exception', e, func, args);
+    return fallback;
+  }
+}
+
+// A "deriver" is a function that never raises; if the underlying function would raise, the
+// corresponding deriver instead returns a fallback value (or `undefined`).
+export function makeDeriver(func, fallback) {
+  return (...args) => tryDerive(args, func, fallback);
+}
+
+// Some widely-used derivers, for convenience.
+export const json = makeDeriver(JSON.parse);