Flexbox - Fill Remaining Space

Fill the remaining height or width in a flex container

Use the flex-grow property to make a flex item consume free space on the main axis.

This property will expand the item as much as possible, adjusting the length to dynamic environments, such as screen re-sizing or the addition / removal of other items.

A common example is flex-grow: 1 or, using the shorthand property, flex: 1.

Hence, instead of width: 96% on your div, use flex: 1.

You wrote:

So at the moment, it's set to 96% which looks OK until you really squash the screen - then the right hand div gets a bit starved of the space it needs.

The squashing of the fixed-width div is related to another flex property: flex-shrink

By default, flex items are set to flex-shrink: 1 which enables them to shrink in order to prevent overflow of the container.

To disable this feature use flex-shrink: 0.

For more details see The flex-shrink factor section in the answer here:

What are the differences between flex-basis and width?

Learn more about flex alignment along the main axis here:

In CSS Flexbox, why are there no "justify-items" and "justify-self" properties?

Learn more about flex alignment along the cross axis here:

How does flex-wrap work with align-self, align-items and align-content?

Getting a flexbox item to fill remaining space

You should use the shorthand flex: 1 on the first child element instead of flex-grow: 1. This sets the other properties intelligently.

Working version:
https://plnkr.co/edit/dT3qxYj7nvYN0xHT4yH8?p=preview

How to have 1 column in a flex box fill remaining space

try adding flex: 1; in #titles.
learn about flex properties over here

#titles {
 flex: 1; 
}

codepen link

CSS: How to make div fill remaining height inside of a flex-child

It would be more helpful when you would of provided your existing CSS to better understand what you are trying to do. However I hope the example below will help you figure out how to solve what you are trying to accomplish.

Html:

<div class="flex-parent-row">
   <div class="flex-child">
      <div class="auto-height"> auto div</div>
      <div class="i-want-this-one-to-fill-remaining-height"> fill remaining div</div>
   </div>
</div>

CSS:

.flex-parent-row {
    display: flex;
    flex-direction: column;
    height: 200px;
    width: 500px;
    border: 1px solid #000;
 }
.flex-child {
    border: 1px solid #000;
    background-color: red;
    display: flex;  
    flex-direction: column;
    flex-grow: 1; 
 }
 .auto-height {
    background: orange;
 }

 .i-want-this-one-to-fill-remaining-height {
    flex-grow: 1;
    background-color: lightblue;
 }

If you need additional help please provide more code.

flexbox layout: take remaining space but do not expand more than 100% of remaining space

You did not close properly your div <div/> instead </div>, Also, less CSS can be used

body{margin:0}

<div style="height: 100vh; display: flex; flex-direction: column;">
   <div>
      This is some fixed heighted content
   </div>
   <div style="flex:1;overflow:auto;">
      A lot of text content here that overflows the remaining space.
      So this div should take up the remaining available space (100vh - space of div above) 
      and the inner content should become scrollable
      But it ends up expanding the div to the text content and thus the whole screen becomes 
      scrollable instead of just the divA lot of text content here that overflows the remaining space.
      So this div should take up the remaining available space (100vh - space of div above) 
      and the inner content should become scrollable
      But it ends up expanding the div to the text content and thus the whole screen becomes 
      scrollable instead of just the divA lot of text content here that overflows the remaining space.
      So this div should take up the remaining available space (100vh - space of div above) 
      and the inner content should become scrollable
      But it ends up expanding the div to the text content and thus the whole screen becomes 
      scrollable instead of just the divA lot of text content here that overflows the remaining space.
      So this div should take up the remaining available space (100vh - space of div above) 
      and the inner content should become scrollable
      But it ends up expanding the div to the text content and thus the whole screen becomes 
      scrollable instead of just the divA lot of text content here that overflows the remaining space.
      So this div should take up the remaining available space (100vh - space of div above) 
      and the inner content should become scrollable
      But it ends up expanding the div to the text content and thus the whole screen becomes 
      scrollable instead of just the divA lot of text content here that overflows the remaining space.
      So this div should take up the remaining available space (100vh - space of div above) 
      and the inner content should become scrollable
      But it ends up expanding the div to the text content and thus the whole screen becomes 
      scrollable instead of just the div
   </div>
</div>

Fill remaining vertical space with CSS using display:flex

Make it simple : DEMO

section {  display: flex;  flex-flow: column;  height: 300px;}
header {  background: tomato;  /* no flex rules, it will grow */}
div {  flex: 1;  /* 1 and it will fill whole space left if no flex value are set to other children*/  background: gold;  overflow: auto;}
footer {  background: lightgreen;  min-height: 60px;  /* min-height has its purpose :) , unless you meant height*/}

<section>  <header>    header: sized to content    <br/>(but is it really?)  </header>  <div>    main content: fills remaining space<br> x    <br>x<br>x<br>x<br>x<br>x<br>x<br>x<br>x<br>x<br>    <!-- uncomment to see it break -->    x<br>x<br>x<br>x<br>x<br>x<br>x<br>x<br>x<br> x    <br>x<br>x<br>x<br>x<br>x<br>x<br>x<br>x<br> x    <br>x<br>x<br>x<br>x<br>x<br>x<br>x<br>x<br> x    <br>x<br>x<br>x<br>x<br>x<br>x<br>x<br>x<br>    <!-- -->  </div>  <footer>    footer: fixed height in px  </footer></section>