Add calc fix & friendlier error message

.eslintrc

@@ -5,6 +5,7 @@
   "rules": {
     "arrow-parens": 0,
     "import/no-extraneous-dependencies": 0,
+    "no-underscore-dangle": 0,
     "react/jsx-filename-extension": 0,
     "react/destructuring-assignment": 0
   }

packages/babel-plugin-bridge/index.js

@@ -0,0 +1,42 @@
+const { existsSync, mkdirSync } = require('fs');
+const { resolve } = require('path');
+const { ncp } = require('ncp');
+const { declare } = require('@babel/helper-plugin-utils');
+
+module.exports = declare((api, opts) => {
+  api.assertVersion(7);
+
+  const modules = opts.modules || 'node_modules';
+  const out = opts.out || 'dist/modules';
+
+  return {
+    visitor: {
+      ImportDeclaration({
+        node: {
+          source: { value },
+        },
+      }) {
+        const outputDir = resolve(process.cwd(), out);
+        if (!existsSync(outputDir)) mkdirSync(outputDir);
+
+        const isLocalModule = value[0] === '.';
+        if (isLocalModule) {
+          const pathToModule = resolve(__dirname, value);
+          const outputPath = resolve(process.cwd(), out, value);
+          console.log(pathToModule, outputPath);
+          if (!existsSync(outputPath)) {
+            ncp(pathToModule, outputPath, err => console.error(err));
+          }
+          return;
+        }
+
+        const pathToModule = resolve(process.cwd(), modules, value);
+        const outputPath = resolve(process.cwd(), out, value);
+        console.log(pathToModule, outputPath);
+        if (!existsSync(pathToModule)) {
+          ncp(pathToModule, outputPath, err => console.error(err));
+        }
+      },
+    },
+  };
+});

src/ScrollAgent.js

@@ -2,6 +2,10 @@ import React from 'react';
 import PropTypes from 'prop-types';
 import observeRect from '@reach/observe-rect';
 
+// Settings
+const REFIRE = 500; // Time in ms to refire calc (takes care of reflow that happens after final animationFrame firing)
+
+// Values
 const TOP = 'top';
 const CENTER = 'center';
 const BOTTOM = 'bottom';
@@ -13,6 +17,9 @@ class ScrollAgent extends React.PureComponent {
   // Memoized container height, to prevent unnecessary recalcs
   _lastH = -1;
 
+  // setTimeout placeholder
+  _lastRecalc = undefined;
+
   // Ref for scrollspy
   wrapper = React.createRef();
 
@@ -68,13 +75,30 @@ class ScrollAgent extends React.PureComponent {
     if (height > 0 && height !== this._lastH) {
       this.handleRecalc();
       this._lastH = height;
+
+      // After last recalculation, wait 500ms and re-fire.
+      // This fixes calc issues on longer pages when animationFrame skips.
+      clearTimeout(this._lastRecalc);
+      this._lastRecalc = setTimeout(() => this.handleRecalc(), REFIRE);
     }
   };
 
   // Handle height recalculation
   handleRecalc = () => {
+    let elements = [];
+
+    try {
+      elements = this.wrapper.current.querySelectorAll(this.props.selector);
+    } catch (err) {
+      console.error(
+        `⚠️ ReactScrollAgent: failed prop \`selector="${
+          this.props.selector
+        }"\`. Must be a valid param for document.querySelectorAll(): https://developer.mozilla.org/en-US/docs/Web/API/Document/querySelectorAll`
+      );
+      console.error(err);
+    }
     this.setState({
-      positions: [...this.wrapper.current.querySelectorAll(this.props.selector)]
+      positions: [...elements]
         .map(node => node.getBoundingClientRect().top + window.scrollY)
         .sort((a, b) => a - b),
     });