How can I make the [Resolution] field of a case page required?

Question

How can I make the [Resolution] field of a case page required? 

Answer

The issue is that an HTML-control of a RICHTEXT type, which is the [Resolution] field, does not support business rule functionality.

Nevertheless, you can use a work-around solution. Override the save() method and verify whether the specified field is populated therein, as follows;

save: function () {
    if (this.get("Solution")) {
        this.callParent(arguments);
    } else {
        this.showInformationDialog("Populate the [Resolution] field");
    }
}