relies on https://github.com/ozoneplatform/owf/issues/2

Pane.js

Ext.define('Ozone.plugins.pane.Pane', {
    extend: 'Ext.AbstractPlugin', 

    init: function(pane) {
        Ext.Function.interceptAfter(pane,"enableWidgetMove",function(){
            this.shimEl.addCls("paneshim-potential-drop");
        });
        Ext.Function.interceptAfter(pane,"disableWidgetMove",function(){
            this.shimEl.removeCls("paneshim-potential-drop");
        });
    }
});

document.write('<style>                                \
.paneshim-potential-drop {                             \
    background: rgba(0,0,0,0.5);                       \
}                                                      \
                                                       \
.paneshim-potential-drop:after {                       \
    content: "Choose a pane";                          \
    text-align: center;                                \
    width: 100px;                                      \
    padding: 40px;                                     \
    position: absolute;                                \
    left: 50%;                                         \
    top: 50%;                                          \
    margin: -50px 0 0 -90px;                           \
    background: #eee;                                  \
    color: #777;                                       \
    border: 1px solid #777;                            \
    border-radius: 10px;                               \
}                                                      \
                                                       \
.highlight-dashboard-designer-drop {                   \
    background: rgba(0,0,0,0.1) !important;            \
}                                                      \
                                                       \
.highlight-dashboard-designer-drop:after {             \
    content: "Select this pane";                       \
    background: #dff0d8;                               \
    color: #468847;                                    \
    border-color: #468847;                             \
}                                                      \
</style>');