Wikimedia Bugzilla is closed!

Because of the padding and borders, the editing textarea is slightly wider than it should be, causing to to overflow out of the bodyContent by 4px.

Putting the textarea in a container div as follows, with css along these lines, should resolve this:

<div id="editBox">
	<textarea .../>
</div>

#wpTextbox1 {
	padding: 0; 
	border: none;
}
#wpEditBox {
	border: solid 1px #c0c0c0; 
	padding: .1em;
}

Note that the WikiEditor extension fixes this bug but introduces a similar one due to its implementation (Bug 40608); this fix should not affect that.