Unintuitve effect of overflow:hidden

I found myself having to fit a textarea into a space which had user-provided CSS (“skins”) applied to it. This worked surprisingly well, but someone found a skin which had a right-floated sidebar which was playing hell with the textarea, since the textarea needed a fixed width and textareas refuse to overlap floats. So the textare got pushed way down, which looked terrible.

(To make things easier to see, the textarea will be background:transparent throughout.)


I looked into absolute positioning to solve my woes, and put the textarea in a position:relative div with an appropriate height, and set the textarea to be position:absolute in the top-left of that div. This got me closer.


After this I just started fiddling with it. It occurred to me to try overflow:hidden on the textarea’s container, which had an effect I did not expect.


So. Overflow:hidden + absolute positioning + floats = floats not interfering with content. I’m sure this follows rationally from the CSS spec, but I totally didn’t expect the effect.

Further experimentation did reveal that it’s not specific to textareas. Any absolutely positioned content will have this effect if its relative container has overflow:hidden.

Identical effect in: FF3, Chrome, and IE8. Not tested elsewhere yet.

You can see all the CSS involved on this demo page.

(@cheald says that it looks like a variant on Pup’s Box Flow Hack. I’m just happy to have independently stumbled onto something that weird.)

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  1. David, the positioning is irrelevant, it’s just the overflow and floats that matter. The overflow:hiddend (or auto or scroll) will also auto-contain internal floats.

    I agree it’s weird, go figure.

    1. The textarea is not floated, and the issue was not the need the contain the textarea inside its container, it was to keep the other floated items from ‘overflowing’ into it. You can look at this as: overflow:hidden + position: relative not only contains floated children but works to keep any external floated elements from overflowing “into” the element.

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