Editor quickTags: when the user selects some text by triple-clicking, then wraps it in a tag, and the last selected char is `\n`, insert the closing tag before the line break.

Props JoshuaGoodwin azaozz.
Fixes #29571.

git-svn-id: https://develop.svn.wordpress.org/trunk@37661 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/quicktags.js

@@ -526,6 +526,11 @@ function edButton(id, display, tagStart, tagEnd, access) {
 		} else if ( canvas.selectionStart || canvas.selectionStart === 0 ) { // FF, WebKit, Opera
 			startPos = canvas.selectionStart;
 			endPos = canvas.selectionEnd;
+
+			if ( startPos < endPos && v.charAt( endPos - 1 ) === '\n' ) {
+				endPos -= 1;
+			}
+
 			cursorPos = endPos;
 			scrollTop = canvas.scrollTop;
 			l = v.substring(0, startPos); // left of the selection