split-buttons/split-button.css
.gui-split-button {
  --theme:        hsl(220 75% 50%);
  --theme-hover:  hsl(220 75% 45%);
  --theme-active: hsl(220 75% 40%);
  --theme-text:   hsl(220 75% 25%);
  --theme-border: hsl(220 50% 75%);
  --ontheme:      hsl(220 90% 98%);
  --popupbg:      hsl(220 0% 100%);

  --border: 1px solid var(--theme-border);
  --radius: 6px;
  --in-speed: 500ms;
  --out-speed: 100ms;

  display: inline-flex;
  border-radius: var(--radius);
  background: var(--theme);
  color: var(--ontheme);
  fill: var(--ontheme);

  touch-action: manipulation;
  user-select: none;
  -webkit-tap-highlight-color: transparent;

  @media (--dark) {
    --theme:        hsl(220 50% 60%);
    --theme-hover:  hsl(220 50% 65%);
    --theme-active: hsl(220 75% 70%);
    --theme-text:   hsl(220 10% 85%);
    --theme-border: hsl(220 20% 70%);
    --ontheme:      hsl(220 90% 5%);
    --popupbg:      hsl(220 10% 30%);
  }

  & button {
    cursor: pointer;
    appearance: none;
    background: none;
    border: none;

    display: inline-flex;
    align-items: center;
    gap: 1ch;
    white-space: nowrap;

    font-family: inherit;
    font-size: inherit;
    font-weight: 500;

    padding-block: 1.25ch;
    padding-inline: 2.5ch;

    color: var(--ontheme);
    outline-color: var(--theme);
    outline-offset: -5px;

    &:is(:hover, :focus-visible) {
      background: var(--theme-hover);
      color: var(--ontheme);

      & > svg {
        stroke: currentColor;
        fill: none;
      }
    }

    &:active {
      background: var(--theme-active);
    }
  }

  & > button {
    border-radius: var(--radius) 0 0 var(--radius);

    @supports (border-start-start-radius: 1px) {
      border-end-start-radius: var(--radius);
      border-start-start-radius: var(--radius);
    }
  }

  @media (--light) {
    & > button,
    & button:is(:focus-visible, :hover) {
      text-shadow: 0 1px 0 var(--theme-active);
    }
    & > .gui-popup-button > svg,
    & button:is(:focus-visible, :hover) > svg {
      filter: drop-shadow(0 1px 0 var(--theme-active));
    }
  }
  
  & svg {
    inline-size: 2ch;
    box-sizing: content-box;
    stroke-linecap: round;
    stroke-linejoin: round;
    stroke-width: 2px;
  }
}

.gui-popup-button {
  inline-size: 4ch;
  cursor: pointer;
  position: relative;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  border-inline-start: var(--border);
  border-radius: 0 var(--radius) var(--radius) 0;

  @supports (border-start-start-radius: 1px) {
    border-inline-start: var(--border);
    border-start-end-radius: var(--radius);
    border-end-end-radius: var(--radius);
  }

  &:is(:hover,:focus-within) {
    background: var(--theme-hover);
  }

  /* fixes iOS trying to be helpful */
  &:focus {
    outline: none;
  }

  &:active {
    background: var(--theme-active);
  }
  
  &:focus-within {
    & > svg {
      transition-duration: var(--in-speed);
      transform: rotateZ(.5turn);
    }
    & > .gui-popup {
      transition-duration: var(--in-speed);
      opacity: 1;
      transform: translateY(0);
      pointer-events: auto;
    }
  }

  @media (--motionOK) {
    & > svg {
      transition: transform var(--out-speed) ease;
    }
    & > .gui-popup {
      transform: translateY(5px);

      transition: 
        opacity var(--out-speed) ease,
        transform var(--out-speed) ease;
    }
  }
}

.gui-popup {
  --shadow: 220 70% 15%;
  --shadow-strength: 1%;

  opacity: 0;
  pointer-events: none;

  position: absolute;
  inset-block-end: 80%;
  inset-inline-start: -1.5ch;
  
  list-style-type: none;
  background: var(--popupbg);
  color: var(--theme-text);
  padding-inline: 0;
  padding-block: .5ch;
  border-radius: var(--radius);
  overflow: hidden;
  display: flex;
  flex-direction: column;
  font-size: .9em;
  transition: opacity var(--out-speed) ease;

  box-shadow:
    0 -2px 5px 0 hsl(var(--shadow) / calc(var(--shadow-strength) + 5%)),
    0 1px 1px -2px hsl(var(--shadow) / calc(var(--shadow-strength) + 10%)),
    0 2px 2px -2px hsl(var(--shadow) / calc(var(--shadow-strength) + 12%)),
    0 5px 5px -2px hsl(var(--shadow) / calc(var(--shadow-strength) + 13%)),
    0 9px 9px -2px hsl(var(--shadow) / calc(var(--shadow-strength) + 14%)),
    0 16px 16px -2px hsl(var(--shadow) / calc(var(--shadow-strength) + 20%))
  ;

  /* fixes iOS trying to be helpful */
  &:focus {outline: none}

  @media (--dark) {
    --shadow-strength: 5%;
    --shadow: 220 3% 2%;

    & button:not(:focus-visible, :hover) {
      text-shadow: 0 1px 0 var(--ontheme);
    }

    & button:not(:focus-visible, :hover) > svg {
      filter: drop-shadow(0 1px 0 var(--ontheme));
    }
  }

  @media (width <= 400px) {
    inset-inline-start: -200%;
  }

  & svg {
    fill: var(--popupbg);
    stroke: var(--theme);

    @media (prefers-color-scheme: dark) {
      stroke: var(--theme-border);
    }
  }

  & button {
    color: var(--theme-text);
    width: 100%;
  }
}
