Symptoms

When editing a form in K2 Designer, on the Layout screen shows the form and then an infinite scrolling starts to happen in just the pane that displays the form. This only happens when editing the form, and not when the form is being run in runtime. This happens particularly when the SharePoint 2013 Theme is applied to the form you are editing.
 

Diagnoses

This has been determined to be a result of a Rich Text Control being applied to the form displaying the behavior.
 

Resolution

The following bug was logged for this behavior:

Bug 543683: K2SF RichTextControl SharePoint2013 Theme with SUMColumn on a view, making use of 100% width causes the form to expand infinitly

The following workaround can be implemented in order to resolve the behavior:

Change the designtime value for "width" on the "rich text control" to 400px. Then, during the form initialize method in your rules set the rich text control properties back to 100%. This will fix the ever-expanding form during design time.