feat: Correctly initialize new restored windows, b=no-bug, c=no-component

src/zen/sessionstore/ZenSessionManager.sys.mjs

@@ -72,10 +72,6 @@ export class nsZenSessionManager {
       compression: SHOULD_COMPRESS_FILE ? 'lz4' : undefined,
       backupFile,
     });
-
-    lazy.SessionStore.promiseAllWindowsRestored.then(() => {
-      delete this._migrationData;
-    });
   }
 
   log(...args) {
@@ -178,6 +174,7 @@ export class nsZenSessionManager {
       // Save the state to the sidebar object so that it gets written
       // to the session file.
       this.saveState(initialState);
+      delete this._migrationData;
       return;
     }
     // If there are no windows, we create an empty one. By default,

src/zen/sessionstore/ZenWindowSync.sys.mjs

@@ -99,7 +99,6 @@ class nsZenWindowSync {
         if (
           window.__SSi &&
           !window.closed &&
-          window.gZenStartup.isReady &&
           !window.gZenWorkspaces?.privateWindowOrDisabled
         ) {
           yield window;