修复：编辑器全屏编辑特定情况下异常问题

Author: git

_examples/completeDemo.html

@@ -85,7 +85,7 @@ <h2>
 
         </script>
     </div>
-    <div id="btns" style="margin-top:20px;">
+    <div id="btns" style="margin-top:20px;height:2000px;">
         <div>
 
             <button onclick="alert(UE.getEditor('editor').getAllHtml())">获得整个html的内容</button>

_src/adapter/editor.js

@@ -608,7 +608,7 @@
             if (this._fullscreen) {
                 var vpRect = uiUtils.getViewportRect();
                 this.getDom().style.cssText =
-                    "border:0;position:absolute;left:0;top:var(--ueditor-top-offset," +
+                    "border:0;position:fixed;left:0;top:var(--ueditor-top-offset," +
                     (this.editor.options.topOffset || 0) +
                     "px);width:" +
                     vpRect.width +