fix #1826

Closes #1826

Merge request studip/studip!1279

resources/vue/components/courseware/CoursewareRibbon.vue

 <template>
     <div :class="{ 'cw-ribbon-wrapper-consume': consumeMode }" :id="isContentBar ? 'contentbar' : null" >
-        <div v-if="stickyRibbon" class="cw-ribbon-sticky-top"></div>
+        <div v-show="stickyRibbon" class="cw-ribbon-sticky-top"></div>
         <header class="cw-ribbon" :class="{ 'cw-ribbon-sticky': stickyRibbon, 'cw-ribbon-consume': consumeMode }">
             <div class="cw-ribbon-wrapper-left">
                 <nav class="cw-ribbon-nav" :class="buttonsClass">