chrome/browser/resources/tab_strip/tab_group.html

<style>
:host {
  --tabstrip-tab-group-title-height: 22px;
  --tabstrip-tab-group-title-margin: var(--tabstrip-tab-spacing);
  --tab-thumbnail-height-in-group: calc(var(--tabstrip-tab-thumbnail-height) -
      var(--tabstrip-tab-group-title-height) -
      var(--tabstrip-tab-group-title-margin));
  --tab-thumbnail-width-in-group: calc(var(--tab-thumbnail-height-in-group) *
      var(--tabstrip-tab-thumbnail-aspect-ratio));
}

:host(:empty:not([dragging])) {
  /* A tab group can temporarily become empty as a tab is being dragged out. */
  display: none;
}

:host([dragged-out_]) {
  display: none;
}

#tabGroup {
  border-radius: 8px;
  box-shadow: 0 0 0 2px rgb(var(--tabstrip-tab-group-color-rgb));
  padding: var(--tabstrip-tab-spacing);
}

#chipContainer {
  min-width: 100%;
  position: relative;
  width: 0;
}

#chip {
  display: inline-block;
  height: var(--tabstrip-tab-group-title-height);
  margin-bottom: var(--tabstrip-tab-group-title-margin);
  max-width: 100%;
}

#chip:focus {
  outline: none;
}

#title {
  background: rgb(var(--tabstrip-tab-group-color-rgb));
  border-radius: 4px;
  box-sizing: border-box;
  color: rgb(var(--tabstrip-tab-group-text-color-rgb));
  display: inline-block;
  height: var(--tabstrip-tab-group-title-height);
  line-height: var(--tabstrip-tab-group-title-height);
  max-width: 100%;
  overflow: hidden;
  padding: 0 6px;
  text-overflow: ellipsis;
  white-space: nowrap;
}

#title:empty {
  border-radius: 50%;
  height: 16px;
  margin-top: 4px;
  width: 16px;
}

#chip:focus #title {
  box-shadow: 0 0 0 2px var(--tabstrip-focus-outline-color);
  outline: none;
}

/*
 * When the drag starts, the tab group will shrink and the tabs will stack
 * together. If the touch position is outside the draggable element the drag
 * will be cancelled. In order to prevent that, the dragHandle is set to the
 * width of the thumbnail to make sure it's still under touch position after
 * the tab group shrinks.
 */
#dragHandle {
  height: 100%;
  position: absolute;
  top: 0;
  width: var(--tab-thumbnail-width-in-group);
}

#tabs {
  display: flex;
  min-width: fit-content;
}

:host ::slotted(tabstrip-tab) {
  --tabstrip-tab-thumbnail-height: var(--tab-thumbnail-height-in-group);
  --tabstrip-tab-thumbnail-width: var(--tab-thumbnail-width-in-group);

  /* Recalculate tab heights and widths off new thumbnail sizes. */
  --tabstrip-tab-height: calc(var(--tabstrip-tab-title-height) +
                              var(--tabstrip-tab-thumbnail-height));
  --tabstrip-tab-width: var(--tabstrip-tab-thumbnail-width);
}

:host([getting-drag-image_]) #dragImage {
  /* Enough padding for the drop shadow on #tabGroup. The position of the
     drag image is translated with the same value as the padding, to ensure
     that the drag image stays in the same position as it would be without
     padding. This is to ensure drag events do not get canceled as drag targets
     move around. */
  --drag-image-padding: 25px;
  padding: var(--drag-image-padding);
  transform: translate(
      calc(var(--drag-image-x-direction, -1) * var(--drag-image-padding)),
      calc(-1 * var(--drag-image-padding)));
}

:host-context([dir='rtl']):host([getting-drag-image_]) #dragImage {
  --drag-image-x-direction: 1;
}

:host([getting-drag-image_]) #tabGroup {
  background: var(--tabstrip-background-color);
  border-radius: var(--tabstrip-tab-border-radius);
  box-shadow: var(--tabstrip-tab-dragging-shadow);
  height: var(--tabstrip-tab-height);
  overflow: hidden;
  transform: scale(var(--tabstrip-tab-drag-image-scale));
}

:host([getting-drag-image_]) ::slotted(tabstrip-tab) {
  --tabstrip-tab-active-border-color: var(--tabstrip-tab-separator-color);
}

:host([getting-drag-image_]) ::slotted(tabstrip-tab:first-child) {
  /* Make sure the first tab is always above the other tabs. */
  z-index: 4;
}

:host([getting-drag-image_]) ::slotted(tabstrip-tab:nth-child(2)),
:host([getting-drag-image_]) ::slotted(tabstrip-tab:nth-child(3)),
:host([getting-drag-image_]) ::slotted(tabstrip-tab:nth-child(4)),
:host([getting-drag-image_]) ::slotted(tabstrip-tab:nth-child(5)) {
  /* Add a negative margin to the 2nd - 5th tab to move them behind the first
     tab to give an impression that they are a stacked group of tabs.
     Transform cannot be used here since we also want the space the tabs
     take up to be moved. */
  margin-inline-start: calc(-1 * var(--tabstrip-tab-width));
}

:host([getting-drag-image_]) ::slotted(tabstrip-tab:nth-child(2)) {
  z-index: 3;
}

:host([getting-drag-image_]) ::slotted(tabstrip-tab:nth-child(3)) {
  z-index: 2;
}

:host([getting-drag-image_]) ::slotted(tabstrip-tab:nth-child(4)) {
  z-index: 1;
}

:host([getting-drag-image_]) ::slotted(tabstrip-tab:nth-child(5)) {
  padding: 0;
  z-index: 0;
}

:host([getting-drag-image_]) ::slotted(tabstrip-tab:nth-child(n + 6)) {
  display: none;
}

:host([dragging]) #dragPlaceholder {
  background: var(--tabstrip-tab-background-color);
  border-radius: var(--tabstrip-tab-border-radius);
  height: var(--tabstrip-tab-height);
  opacity: 0.5;
  width: var(--tabstrip-tab-width);
}

:host([dragging]) #dragImage {
  /* Position the actual drag image out of view so it is not visible. */
  position: absolute;
  top: calc(100vh + var(--drag-image-padding));
}

:host([touch_pressed_]) #dragImage {
  transform: scale(1.1);
  transform-origin: center;
  transition-duration: 300ms;
}
</style>

<div id="dragPlaceholder"></div>

<div id="dragImage">
  <div id="tabGroup">
    <div id="chipContainer">
      <div id="chip" tabindex="0" role="button"
         aria-describedby="tabs">
        <div id="title"></div>
        <div id="dragHandle" draggable="true"></div>
      </div>
    </div>
    <div id="tabs">
      <slot></slot>
    </div>
  </div>
</div>