fix(mdviewer): deactivate MarkdownSync when hiding panel to fix reopen

When blanking the iframe on panel hide, deactivate MarkdownSync and
reset _isMdviewrActive so the next panel show properly reuses the
persistent md iframe instead of activating on the blank iframe.

Author: abose

src/extensionsIntegrated/Phoenix-live-preview/main.js

@@ -602,6 +602,8 @@ define(function (require, exports, module) {
         // src. so we delete the node itself to eb thorough.
         // Don't destroy the persistent md iframe — just hide it
         if ($mdviewrIframe && $iframe[0] === $mdviewrIframe[0]) {
+            MarkdownSync.deactivate();
+            _isMdviewrActive = false;
             $mdviewrIframe.hide();
             let newIframe = $(LIVE_PREVIEW_IFRAME_HTML);
             $mdviewrIframe.after(newIframe);