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HTML 

              
                <div class="center">

<div class="splitbutton">
<input type="button" value="Primary" onclick="alert('primary');" />
<select onchange="alert(this.value); this.value='primary';">
  <option value="primary" hidden>Primary</option>
  <option value="secondary">Secondary</option>
  <option value="alternative">Alternative</option>
</select>
</div>

  
  <p><a href="https://web.dev/shows/gui-challenges/Qcpru-fIgwk/">GUI challenge?</a><br />Maybe...</p>
</div>
!

CSS 

              
                body {
  background: #f5f8f9;
  font-family: sans-serif;
  line-height: 1.5rem;
}
.center {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}
.center p, .center p a {color: gray;}
.splitbutton {
  position: relative; 
  display: inline-block;
}
.splitbutton::before,
.splitbutton::after {
  content: "";
  position: absolute;
  right: 0.7rem;
  top: 1.1rem;
  width: 0;
  height: 0;
  border-top: 0.35rem solid white;
  border-left: 0.35rem solid transparent;
  border-right: 0.35rem solid transparent;
  pointer-events: none;
}
.splitbutton::before {
  border-top-color: hsl(220, 75%, 40%);
  top: calc(1.1rem + 1px);
}
.splitbutton input, .splitbutton select {
  background: #2060df; 
  color: white;
  border: 0;
  height: 2.5rem;
  line-height: 2.5rem;
  font-family: sans-serif;
  border-top-left-radius: 0.3rem;
  border-bottom-left-radius: 0.3rem;
  cursor: pointer;
}
.splitbutton input {
  width: calc(100% - 2rem);
  position: absolute;
  border-right: 1px solid #9fb5df;
  text-shadow: 0 1px 0 hsl(220, 75%, 40%);
}
.splitbutton input:hover,
.splitbutton select:hover {
  background: hsl(220, 75%, 45%);
}
.splitbutton input:active,
.splitbutton select:active {
  background: hsl(220, 75%, 40%)
}
.splitbutton select {
  padding-right: 2rem;
  -webkit-appearance: none;
  -moz-appearance: none;
  appearance: none;
  border-top-right-radius: 0.3rem;
  border-bottom-right-radius: 0.3rem;
}
!

JS 

              
                /* 

Hey Adam, do you really need a split button? Isn't hiding things under a tiny non-descriptive arrow a clear sign of bad interface design? However, when you think you really do... I would not overthink it. You can just use native browser controls. They work really well, also on mobile, AND they are accessible by default. I hope I did not spoil this challenge for you. I am looking forward to your next GUI challenge. Have a great day!

https://web.dev/shows/gui-challenges/Qcpru-fIgwk/

*/
!
999px

Console