Thanks. It's more than just the header adding height.

Here's the markup:

<ui:composition xmlns="http://www.w3.org/1999/xhtml"
                xmlns:ui="http://java.sun.com/jsf/facelets"
                xmlns:h="http://java.sun.com/jsf/html"
                xmlns:f="http://java.sun.com/jsf/core"
                xmlns:a="http://richfaces.org/a4j"
                xmlns:rich="http://richfaces.org/rich">


<h:form style="margin: 0;">
    <rich:popupPanel id="imageModalPanel" autosized="true"
                     minWidth="425" minHeight="500"
                     moveable="true" top="50"
                     show="#{instance.getCurrentPhotoId() != 0}"
                     domElementAttachment="parent">
        <f:facet name="header">
            <h:panelGroup styleClass="widget-details-section-header">
                <div class="widget-details-section-header-left">
                    <h:outputText value="Photo:  #{instance.name}"/>
                </div>
            </h:panelGroup>
        </f:facet>
        <f:facet name="controls">
            <h:panelGroup>


                <a:commandLink execute="@this" id="hideFullPhoto"
                               actionListener="#{instance.setCurrentPhotoId(0)}"
                               title="#{messages['close']}" styleClass="close-button-icon"
                               render="imageModalPanel"/>


            </h:panelGroup>
        </f:facet>
        <a:mediaOutput id="fullPhoto" element="img"
                       createContent="#{imageLoader.paintImageFullPhoto}"
                       value="#{instance.currentPhotoId}"
                       styleClass="widget-photo">
            <a:ajax event="click"
                    listener="#{instance.setCurrentPhotoId(0)}"
                    render="imageModalPanel"/>
        </a:mediaOutput>
    </rich:popupPanel>
</h:form>

(Weird. Could see it in editor but not in my reply. Edited to try XML...)

And paintImageFullPhoto ends up here:

public void paintImage(
    final OutputStream out,
    final Object data,
    int imageWidth
) throws IOException {


    if (null == data || !(data instanceof Long)) {
        return;
    }


    final Long mediaId = (Long) data;
    String photoPath = getPhotosPath(mediaId, imageWidth);


    if (StringUtils.isNullOrEmpty(photoPath)) {
        return;
    }


    byte[] content = null;


    try {
        content = getFileContents(photoPath);
    } catch (FileNotFoundException e) {
        LOG.error("Couldn't find photo from path: " + photoPath + "; " + e.getMessage());
    } catch (IOException e) {
        LOG.error("Error loading photo data for: " + photoPath + "; " + e.getMessage());
    }


    if (content != null) {
        out.write(content);
    }
}

And getFileContents is essentially:

String fileUrl = getPhotoServerUrlBackend() + filePath;


try {
    URL urlPhotos = new URL(fileUrl);
    content = getUrlContents( urlPhotos );
} catch (Exception e) {
    LOG.info(
        "Error getting content from url: " + fileUrl + " " + e.getMessage()
    );
}

Not sure if you need more than that, and also note that I'm updating some old RF3.3.3 stuff so maybe I've missed something with those changes.