callback
The command procedure to be executed when panel has received the keyboard focus.
This element applies only to panels of types text and combo_box. The window parent of panel and panel itself are passed to the procedure as parameters. When panel belongs to a table window, the row/column index of the panel is passed as a third parameter.
Initially, panels have a .before procedure of null.
panel.before (<window>,<panel>{,<row>})
mywin.textinput.before = { args w=window, p=panel # Set a default value in the text box p.tvalue = '100' }