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Every element that is selected will get this event, you can return true if you want the user to be able to select it, or false if you don't want that the users uses that element in clientDesign mode. It is better practice to set your element not selectable with the client design properties before you go into clientDesign mode because then the element is really not selectable and if you use this method you could see a second of selection. 

onResize

Handle the resize of a element. This is a good position to save the element sizes.

Client design properties

Wiki Markup//by default an element with an name is selectable in client designThere are two client design properties selectable and handles.

Selectable:

By default an element with a name is selectable in client design mode but you can set element to not be selectable.

Sample:

elements\['element_1'\].putClientProperty(CLIENTDESIGN.SELECTABLE, false);