CSS problems on Contact Notes View modal dialog
Turns out it was a CSS conflict with the Woocommerce Event Manager plugin in WordPress. In fact it was affecting the formatting of many different modal dialog windows in CiviCRM, not just the Contact Notes dialog.
In case anyone's interested, I tracked it down by opening the misbehaving dialog in CiviCRM, then used Chrome -&gt; Developer Tools -&gt; Elements to identify the div with the right-shift problem. The CSS hierarchy for this element showed (in the 'Styles' pane) that the CSS heirarchy was interrupted (Jquery UI classes were struckthrough), and it gave the CSS filename, which gave a clue to the identity of the naughty WP plugin.
Of course, I could have just deactivated each WP plugin in turn...