Editor: fix `Resizer.js` error. (#26703)

* Editor: fix `Resizer.js` error.

* Update main.css

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Co-authored-by: Michael Herzog <[email protected]>

editor/js/Resizer.js

@@ -36,7 +36,7 @@ function Resizer( editor ) {
 
 		const cX = clientX < 0 ? 0 : clientX > offsetWidth ? offsetWidth : clientX;
 
-		const x = offsetWidth - cX;
+		const x = Math.max( 260, offsetWidth - cX ); // .TabbedPanel min-width: 260px
 
 		dom.style.right = x + 'px';