Work-around for Youtube video elements positioned off-display at replacement time.

src/content/PlaceHolder.js

@@ -55,6 +55,9 @@ var PlaceHolder = (() => {
     for (let p of props) {
       destStyle[p] = srcStyle[p];
     }
+    if (src.offsetTop < 0 && src.offsetTop <= (-src.offsetHeight)) {
+      destStyle.top = "0"; // fixes video player off-display position on Youtube
+    }
     destStyle.display = srcStyle.display !== "block" ? "inline-block" : "block";
   }
 

src/content/media.js

@@ -35,13 +35,11 @@ if ("MediaSource" in window) ns.on("capabilities", event => {
     notifyPage();
 
     if (mediaBlocker) {
-      (async () => {
+      let exposedMime = `${mime} (MSE)`;
+      setTimeout(() => {
         let me = Array.from(document.querySelectorAll("video,audio"))
           .find(e => e.srcObject === ms || urls && urls.has(e.src));
-
         if (!me) return;
-        let exposedMime = `${mime} (MSE)`;
-
         try {
           let ph = PlaceHolder.create("media", request);
           ph.replace(me);
@@ -49,7 +47,7 @@ if ("MediaSource" in window) ns.on("capabilities", event => {
         } catch (e) {
           error(e);
         }
-      })();
+      }, 0);
       throw new Error(`${exposedMime} blocked by NoScript`);
     }