JSFiddle - React, Tailwind, and code Playground

HTML

<div class="sidebar">
    <div class="container">
        <div class="top">
            <h3>Title</h3>
            <img src="http://dummyimage.com/200x100/000/fff" alt="The size of this can change" />
        </div>
        
        <div class="middle">
            <!-- there is an unknown amount of content here represented below coming from a server -->
            <h2>There may be very few of these or a lot of them</h2>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <p>see the heading also in this div ASDFasdf asdf asdf asdf asdf asdf</p>
            <h2>There may be very few of these or a lot of them</h2>
            <h2>There may be very few of these or a lot of them</h2>
            <h2>There may be very few of these or a lot of them</h2>
            <h2>There may be very few of these or a lot of them</h2>
            <h2>There may be very few of these or a lot of them END.</h2>
        </div>
  ...

CSS

.sidebar {
    position:fixed;
    padding-top: 167px; /* add .bottom height (100px) */
    height: 100%;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
    width: 20%;
    bottom: 0;
    right: 20px;
}
.container {
    position: relative;
    height: 100%;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
    margin-top: -100px; /* set to .bottom height */
    overflow: auto;
}
.bottom {
    height: 100px;
}