How to Dynamically Extend Aside Background Color to Fill Content Area?

Force Aside Element to Dynamically Fill Content Area with Background Color

In HTML, a common layout involves an aside element alongside a main content area within a scrolling overflow. However, the aside element's background color typically truncates at the visible area, leaving a blank space below the fold. This article addresses the challenge of extending the aside background color dynamically to match the total content height.

Problem Statement

Consider the following HTML structure:

<div id="body">
  <main>...</main>
  <aside>...</aside>
</div>

The #body element has an overflow set to auto, enabling scrolling of the main content. The aside element should have its background color stretch vertically from the top to the bottom of the total content, even though the rendered height is limited to the visible area.

Limitations

• No additional elements can be added.
• The aside element must scroll with the main content.
• It must have its background color extend to the full content height below the fold.
• The aside element cannot have its overflow set to auto.

JavaScript Solution

Unfortunately, this cannot be achieved solely through CSS. The overflow property affects content, not the background. Therefore, a JavaScript solution is necessary. One approach involves:

1. Create a function to set the aside height:

function setAsideHeight() {
  const bodyHeight = document.getElementById("body").offsetHeight;
  const aside = document.querySelector("aside");
  aside.style.height = bodyHeight + "px";
}

2. Call the function when the window loads and resizes:

window.addEventListener("load", setAsideHeight);
window.addEventListener("resize", setAsideHeight);

This script dynamically adjusts the aside height to match the total content height, ensuring that its background color extends to the bottom of the scrolling area.

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