chrome/browser/resources/bookmarks/app.html

<style>
  :host {
    color: var(--cr-primary-text-color);
    display: flex;
    flex-direction: column;
    height: 100%;
    line-height: 1.54;  /* 20px. */
  }

  #main-container {
    display: flex;
    flex-direction: row;
    flex-grow: 1;
    overflow: hidden;
  }

  #splitter {
    box-sizing: border-box;
    cursor: col-resize;
<if expr="is_win">
    /* Unlike other platforms, e-resize is a two-way arrow on Windows. */
    cursor: e-resize;
</if>
    flex: 0 0 var(--splitter-width);
    opacity: 0;
  }

  #splitter:hover,
  #splitter.splitter-active {
    border-inline-start: 1px solid rgba(0, 0, 0, 0.1);
    opacity: 1;
    transition: opacity 100ms ease-out;
  }

  @media (prefers-color-scheme: dark) {
    #splitter:hover,
    #splitter.splitter-active {
      border-inline-start-color: var(--cr-separator-color);
    }
  }

  #sidebar {
    display: flex;
    flex-direction: column;
    max-width: 40%;
    min-width: var(--min-sidebar-width);
    width: var(--min-sidebar-width);
  }

  bookmarks-folder-node {
    flex: 1;
    overflow-y: auto;
    padding: 8px 0;
    user-select: none;
  }

  managed-footnote {
    --managed-footnote-icon-padding: 12px;
    flex: 0;
    padding-bottom: 24px;
    padding-top: 24px;
  }

  bookmarks-list {
    flex: 1;
    overflow-x: hidden;
  }
</style>
<bookmarks-toolbar sidebar-width="[[sidebarWidth_]]" role="banner">
</bookmarks-toolbar>
<div id="main-container">
  <div id="sidebar" role="tree" aria-label="$i18n{sidebarAxLabel}">
    <bookmarks-folder-node item-id="0" depth="-1"></bookmarks-folder-node>
    <managed-footnote></managed-footnote>
  </div>
  <cr-splitter id="splitter"></cr-splitter>
  <bookmarks-list></bookmarks-list>
</div>
<bookmarks-router></bookmarks-router>
<bookmarks-command-manager></bookmarks-command-manager>
<cr-toast-manager duration="10000">
  <dom-if>
    <template>
      <cr-button on-click="onUndoClick_" aria-label="$i18n{undoDescription}">
        $i18n{undo}
      </cr-button>
    </template>
  </dom-if>
</cr-toast-manager>