What you intend to do can be done using CSS flexbox—there is no need to use CSS grid. The bad news is that HTML + CSS is not so smart that declaring resize and draggable will make the layout flexible and adjustable by user interaction. For that, you will have to use JS. The good news is that this is actually not too complicated.
Here is a quick screen grab of output the code below:
However, for you to understand the code I will post below, you will have to familiarize yourself with:
- Event binding using
.addEventListener. In this case, we will use a combination of mousedown, mouseup and mousemove to determine whether the user is in the middle of dragging the element
- CSS flexbox layout
Description of the solution
Initial layout using CSS
Firstly, you will want to layout your boxes using CSS flexbox. We simply declare display: flex on the parent, and then use flex: 1 1 auto (which translates to "let the element grow, let the element shrink, and have equal widths). This layout is only valid at the initial rendering of the page:
.wrapper {
/* Use flexbox */
display: flex;
}
.box {
/* Use box-sizing so that element's outerwidth will match width property */
box-sizing: border-box;
/* Allow box to grow and shrink, and ensure they are all equally sized */
flex: 1 1 auto;
}
Listen to drag interaction
You want to listen to mouse events that might have originated from your .handler element, and you want a global flag that remembers whether the user is dragging or not:
var handler = document.querySelector('.handler');
var isHandlerDragging = false;
Then you can use the following logic to check if the user is dragging or not:
document.addEventListener('mousedown', function(e) {
// If mousedown event is fired from .handler, toggle flag to true
if (e.target === handler) {
isHandlerDragging = true;
}
});
document.addEventListener('mousemove', function(e) {
// Don't do anything if dragging flag is false
if (!isHandlerDragging) {
return false;
}
// Set boxA width properly
// [...more logic here...]
});
document.addEventListener('mouseup', function(e) {
// Turn off dragging flag when user mouse is up
isHandlerDragging = false;
});
Computing the width of box A
All you are left with now is to compute the width of box A (to be inserted in the [...more logic here...] placeholder in the code above), so that it matches that of the movement of the mouse. Flexbox will ensure that box B will fill up the remaining space:
// Get offset
var containerOffsetLeft = wrapper.offsetLeft;
// Get x-coordinate of pointer relative to container
var pointerRelativeXpos = e.clientX - containerOffsetLeft;
// Resize box A
// * 8px is the left/right spacing between .handler and its inner pseudo-element
// * Set flex-grow to 0 to prevent it from growing
boxA.style.width = (pointerRelativeXpos - 8) + 'px';
boxA.style.flexGrow = 0;
Working example
var handler = document.querySelector('.handler');
var wrapper = handler.closest('.wrapper');
var boxA = wrapper.querySelector('.box');
var isHandlerDragging = false;
document.addEventListener('mousedown', function(e) {
// If mousedown event is fired from .handler, toggle flag to true
if (e.target === handler) {
isHandlerDragging = true;
}
});
document.addEventListener('mousemove', function(e) {
// Don't do anything if dragging flag is false
if (!isHandlerDragging) {
return false;
}
// Get offset
var containerOffsetLeft = wrapper.offsetLeft;
// Get x-coordinate of pointer relative to container
var pointerRelativeXpos = e.clientX - containerOffsetLeft;
// Arbitrary minimum width set on box A, otherwise its inner content will collapse to width of 0
var boxAminWidth = 60;
// Resize box A
// * 8px is the left/right spacing between .handler and its inner pseudo-element
// * Set flex-grow to 0 to prevent it from growing
boxA.style.width = (Math.max(boxAminWidth, pointerRelativeXpos - 8)) + 'px';
boxA.style.flexGrow = 0;
});
document.addEventListener('mouseup', function(e) {
// Turn off dragging flag when user mouse is up
isHandlerDragging = false;
});
body {
margin: 40px;
}
.wrapper {
background-color: #fff;
color: #444;
/* Use flexbox */
display: flex;
}
.box {
background-color: #444;
color: #fff;
border-radius: 5px;
padding: 20px;
font-size: 150%;
/* Use box-sizing so that element's outerwidth will match width property */
box-sizing: border-box;
/* Allow box to grow and shrink, and ensure they are all equally sized */
flex: 1 1 auto;
}
.handler {
width: 20px;
padding: 0;
cursor: ew-resize;
flex: 0 0 auto;
}
.handler::before {
content: '';
display: block;
width: 4px;
height: 100%;
background: red;
margin: 0 auto;
}
<div class="wrapper">
<div class="box">A</div>
<div class="handler"></div>
<div class="box">B</div>
</div>
