Try use a mutation observer

Instead of every scrolling action, which made scrolling slow on some pages.
This will probably be slow on pages with many DOM changes, so not ideal either..

src/content_script.js

@@ -50,27 +50,53 @@ function hideBanners() {
   return revertSteps
 }
 
-function main() {
+async function main() {
   let revertSteps = []
 
   // XXX We read the location only once, but it may be changed by page scripts.
   // Fortunately we only need the origin, which should remain the same.
   const url = window.location.href
 
-  async function run() {
-    const enabled = await isEnabled(url)
-    if (enabled && window.scrollY > 20) {
+  function run() {
+    if (enabled && scrolledDown) {
       // Hide everything that looks annoying. Remember the steps to revert this.
       revertSteps = hideBanners().concat(revertSteps)
-    } else if (!enabled || window.scrollY <= 0) {
+    } else {
       // Unhide everything we have hidden.
-      revertSteps.forEach(step => step())
-      revertSteps = []
+      if (revertSteps.length > 0) {
+        revertSteps.forEach(step => step())
+        revertSteps = []
+      }
     }
   }
 
-  window.addEventListener('scroll', throttle(run, 300))
-  onEnabledChange(url, run)
+  // Run whenever the extension is enabled/disabled for this domain.
+  let enabled = await isEnabled(url)
+  onEnabledChange(url, newValue => { enabled = newValue; run() })
+
+  // Run whenever crossing the scroll threshold.
+  let scrolledDown = window.scrollY > 20
+  window.addEventListener('scroll', () => {
+    // We add a hysteresis of 20px.
+    if (window.scrollY > 20 && !scrolledDown) {
+      scrolledDown = true
+      run()
+    } else if (window.scrollY <= 0 && scrolledDown) {
+      scrolledDown = false
+      run()
+    }
+  })
+
+  // Run on any page mutation, but at most once per second.
+  const observer = new MutationObserver(throttle(() => run(), 1000))
+  observer.observe(document, {
+    subtree: true,
+    childList: true,
+    attributes: true,
+  })
+
+  // Run once now.
+  run()
 }
 
 main()