The resizable feature seems to be running properly. The problem is caused by how the ace:panel is rendered. The ace:panel markup has a DIV as root element and another enclosed DIV as the content are for the panel. When the resizable feature is use the root DIV for the containment calculation, while its resized content is enclosed by the content DIV. Since both the root DIV and content _DIV have some padding that will shift the resizable ace:panel's content.
Modifying the following CSS classes will prove that the resize feature works well:
ui-panel { padding: 0; }
ui-panel-content { padding: 0; }
or replace the enclosing ace:panel with a DIV:
<div style="width:300px;height:150px; border: 1px solid black;"> ... </div>
Issue is still reproducible on ICEfaces4 trunk r.48632, or EE4 trunk r48694.