JSFiddle - React, Tailwind, and code Playground

HTML

<div class="site-container">
  <div class="site-header">
    <div style="height: 40px;background-color:YellowGreen">My Header Test </div>
  </div>
  <div class="site-body">
    <div class="body-container">
      <div class="site-sidebar-left">
        <div style="background-color:lightgray;">
          left1
          <br /> left2
          <br /> left3
          <br /> left4
          <br /> left5
          <br /> left6
          <br /> left7
          <br /> left8
          <br /> left9
          <br /> left10
          <br />
        </div>
      </div>
      <div class="site-content">
        blah1
        <br /> blah2
        <br /> blah3
        <br /> blah4
        <br /> blah5
        <br /> blah6
        <br /> blah7
        <br /> blah8
        <br /> blah9
        <br /> blah10
        <br /> blah11
        <br /> blah12
        <br /> blah13
        <br /> blah14
        <br /> blah15
        <br /> blah16
        <br /> blah17
        <br /> blah18
        <br /> blah19
        <br /> blah20
        <br /> blah21
        <br /> blah22
        <br /> blah23
        <br /> blah24
        <br /> blah25
        <br /> blah26
        <br /> blah27
        <br /> blah28
        <br /> blah29
        <br /> blah30
        <br /> blah31
        <br /> blah32
        <br /> blah33
        <br /> blah34
        <br /> blah35
        <br /> blah36
        <br /> blah37
        <br /> blah38
        <br /> blah39
        <br /> blah40
        <br /> blah41
        <br /> blah42
        <br /> blah43
        <br /> blah44
        <br /> blah45
        <br /> blah46
        <br /> blah47
        <br /> blah48
        <br /> blah49
        <br /> blah50
        <br /> blah51
        <br /> blah52
        <br /> blah53
        <br /> blah54
        <br /> blah55
        <br /> blah56
        <br /> blah57
        <br /> blah58
        <br /> blah59
        <br /> blah60
        <br /> blah61
        <br /> blah62
        <br /> blah63
        <br /> blah64
       ...

CSS

.site-container {
  display: flex;
  flex-direction: column;
  height: 100vh;
}

.site-body {
  min-height: 0px;
  height: 100%;
}

.body-container {
  display: flex;
  height: 100%;
}

.site-content {
  flex: 1;
  overflow: auto;
}

body {
  margin: 0;
}