Why Does My Fixed Header Move Down, and How Can I Fix It?

Header Moves Down When I Set It Fixed

Here, you're experiencing an issue known as margin collapsing. When you made the header position:fixed, it was removed from the normal flow of the document, making the top margin of the following element (in this case, the form) collapse with the top margin of the body. It's the default behavior of HTML.

To avoid this issue and preserve the vertical spacing between the header and the form, you have two options:

1. Disable Margin Collapsing:

   Add padding-top: 1px; to the body of your CSS to prevent margin collapsing. This small amount of padding effectively breaks the collapse and allows the margin of the form to take effect.

2. Set Top Margin:

   Add top: [desired value]; to the CSS for your header to explicitly set its vertical position relative to the top of the viewport. This will override the collapsed margin and ensure your header remains in the desired position.

Updated Code (Option 1):

body {
  padding-top: 1px; /* disable margin collapsing */
}

Updated Code (Option 2):

#header {
  top: 3rem; /* desired vertical position */
}

Note:

If you are using a framework like Bootstrap or Materialize CSS, they may already have rules in place to handle margin collapsing. In that case, you may need to override those rules or use alternative approaches to achieve the desired positioning.

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