Only setup sortable row wrapper and invisible row placeholders when adding a row to the stage (25a27e6)
Use last-of-type instead of last-child due to drag/drop placeholder occupying last-child position (dc4f312)
Use the pointer to for sortable intersect detection when dragging controls from the control panel onto a bootstrap row. jQuery.ui sortable used tolerance: intersect which caused the placeholder in a row to jump around when near the edges and the new control is less than 50% contained. (995fc2e)
When adding rows, reset the hidden flag for all placeholders (939341c)