My markup has the following structure:
<Header/>
<main style={{display: 'flex'}}>
   <div style={{flexGrow: 1}} id='one'>Left Panel</div>
   <div style={{flexGrow: 5}} id='two'>Main Content</div>  
</main>
Do not bother about <Header />. It is a React component that lies at the top of the page and has a certain height (not fixed). What I want is div one to take the entire height of the available viewport; that means 100vh minus the height of the header that we do not know. How is that possible?
 
    