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.

Version: unspecified
Severity: trivial