bug fix: 光标移出需要朗读的元素，又移回来，朗读。光标在需要朗读的元素内部各子孙元素间移动，只朗读一次。

web/src/utils.js

@@ -77,11 +77,13 @@ function extractTextForMouseOver(e) {
   while (!meaningfulNode.getAttribute || !meaningfulNode.getAttribute('tabindex')) {
     meaningfulNode = meaningfulNode.parentNode
     if (!meaningfulNode) {
+      lastMeaningfulNode = null
       return
     }
   }
 
   if (meaningfulNode.getAttribute('tabindex') === '-1') {
+    lastMeaningfulNode = null
     return
   }
 
@@ -91,6 +93,7 @@ function extractTextForMouseOver(e) {
     meaningfulNode.getAttribute('data-aria-viewport-area') !== null ||
     meaningfulNode.getAttribute('data-aria-interaction-area') !== null
   ) {
+    lastMeaningfulNode = null
     return
   }