It is certainly possible to do this without an extra div. Use css and ::after to create the border and change the cursor. Use MouseEvent.offsetX to determine whether to process a click in the element.
In your example, you want a click on the main div, but only on the first 4 pixels. You can do that by checking for e.offsetX < 4 in your click handler:
const BORDER_SIZE = 4;
const panel = document.getElementById("right_panel");
let m_pos;
function resize(e){
const dx = m_pos - e.x;
m_pos = e.x;
panel.style.width = (parseInt(getComputedStyle(panel, '').width) + dx) + "px";
}
panel.addEventListener("mousedown", function(e){
if (e.offsetX < BORDER_SIZE) {
m_pos = e.x;
document.addEventListener("mousemove", resize, false);
}
}, false);
document.addEventListener("mouseup", function(){
document.removeEventListener("mousemove", resize, false);
}, false);
#right_panel {
position: absolute;
width: 96px;
padding-left: 4px;
height: 100%;
right: 0;
background-color: #f0f0ff;
}
#right_panel::after {
content: '';
background-color: #ccc;
position: absolute;
left: 0;
width: 4px;
height: 100%;
cursor: ew-resize;
}
<body>
<div id="right_panel"></div>
</body>
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