fix(editor): Restrict selected cell count in this editor instance

The menububble should not be hidden when any other editor instance has selected cells, in case there are more editors in the same page.

src/components/QuasarTiptap.vue

@@ -210,7 +210,7 @@ export default {
           this.$emit('update', { getJSON, getHTML, state, transaction })
         },
         onTransaction: ({ getJSON, getHTML, state, transaction }) => {
-          const selectedCellElements = document.querySelectorAll('.selectedCell')
+          const selectedCellElements = this.editor.view.dom.querySelectorAll('.selectedCell')
           this.selectedCellSize = selectedCellElements.length
 
           this.$emit('transaction', { getJSON, getHTML, state, transaction })