BookStack/resources/assets/sass/_tinymce.scss

//.mce-container {
//  .mce-menubar, .mce-toolbar-grp {
//    position: fixed;
//    width: 100%;
//  }
//  .mce-container-body {
//    position: relative;
//    overflow: hidden;
//    display: block;
//  }
//}

.mce-tinymce.mce-container.fullscreen {
  position: fixed;
  top: 0;
  height: 100%;
  width: 825px;
  max-width: 100%;
  margin-left: -$-s;
  box-shadow: 0 0 4px 2px rgba(0, 0, 0, 0.08);
}


.mce-tinymce {
  .mce-panel {
    background-color: #FFF;
  }
  .mce-btn {
    background-color: #FFF;
  }
}
.mce-tinymce.mce-container.mce-panel {
  height: 100%;
  max-height: 100%;
  flex: 1;
  display: flex !important;
  flex-direction: column;
  align-items: stretch;
  margin: 0 -1px;
  > .mce-container-body {
    flex: 1;
    display: flex !important;
    flex-direction: column;
    align-items: stretch;
    > .mce-edit-area {
      flex: 1;
      display: flex !important;
      flex-direction: column;
      align-items: stretch;
      iframe {
        flex: 1;
      }
    }
  }
}