Fixed multiple weird errors connected with Selection::extend.

Author: scofalik

core/selection.js

@@ -215,12 +215,20 @@
 
 	function moveNativeSelectionToBookmark( document, bm ) {
 		var sel = document.getSelection(),
-			range = document.createRange();
+			range = document.createRange(),
+			wasZWS = ( bm[ 0 ].node.textContent == '\u200B' );
 
 		range.setStart( bm[ 0 ].node, bm[ 0 ].offset );
 		range.collapse( true );
 		sel.removeAllRanges();
 		sel.addRange( range );
+
+		// For some unknown reason, Selection::addRange clears text node content
+		// if the only character in the content was ZWS. So to prevent
+		// errors, we recreate that. (#13284, #13389, #13307)
+		if ( wasZWS && ( bm[ 0 ].node == bm[ 1 ].node ) && bm[ 0 ].node.textContent.length === 0 )
+			bm[ 1 ].node.textContent = '\u200B';
+
 		sel.extend( bm[ 1 ].node, bm[ 1 ].offset );
 	}