bookstack/resources/assets/sass/_tinymce.scss

.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-container-body.mce-flow-layout {
  text-align: center;
}

.edit-area.flex > .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;
      }
    }
  }
}