opencode/packages/app/src/components/titlebar.css

@keyframes titlebar-tab-fade-left {
  from {
    visibility: hidden;
  }
  to {
    visibility: visible;
  }
}

@keyframes titlebar-tab-fade-right {
  from {
    visibility: visible;
  }
  to {
    visibility: hidden;
  }
}

/* The narrow ranges preserve the binary thresholds of scrollLeft > 0 and
   remaining scroll distance > 1px. Hidden defaults make an inactive timeline
   (no overflow) a no-op. Unsupported browsers also degrade to no fades. */
[data-slot="titlebar-tabs"] {
  timeline-scope: --titlebar-tabs-scroll;

  [data-slot^="titlebar-tabs-fade-"] {
    visibility: hidden;
  }
}

@supports (animation-timeline: --titlebar-tabs-scroll) and (timeline-scope: --titlebar-tabs-scroll) {
  [data-slot="titlebar-tabs-scroll"] {
    scroll-timeline: --titlebar-tabs-scroll x;
  }

  [data-slot="titlebar-tabs-fade-left"] {
    animation: titlebar-tab-fade-left linear both;
    animation-timeline: --titlebar-tabs-scroll;
    animation-range: 0 0.1px;
  }

  [data-slot="titlebar-tabs-fade-right"] {
    animation: titlebar-tab-fade-right linear both;
    animation-timeline: --titlebar-tabs-scroll;
    animation-range: calc(100% - 1.1px) calc(100% - 1px);
  }
}