Reports

Please tell me if this is what you want. If it is, I will edit answer to include description and docs source.

.parent {
  display: flex;
}

.child1 {
  background: #ddd;
  padding: 1rem;
}

.child2 {
  background: #eee;
  padding: 1rem;
  display: flex;
  flex-direction: column;
}

.timeslots {
  flex-basis: 0;
  flex-grow: 1;
  overflow-y: auto;
}

<div class="parent">
  <div class="child1">
    Tall content here (sets height)<br>
    Tall content here (sets height)<br>
    Tall content here (sets height)<br>
  </div>
  <div class="child2">
    <div class="timeslots">
      Lots of scrollable content here...<br>
      Lots of scrollable content here...<br>
      Lots of scrollable content here...<br>
      Lots of scrollable content here...<br>
      Lots of scrollable content here...<br>
      Lots of scrollable content here...<br>
      Lots of scrollable content here...<br>
      Lots of scrollable content here...<br>
    </div>
  </div>
</div>

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