Detecting folder page focus changes

When a folder page is selected by the user, and the focus is in the context of that folder, the focus is set to the field with the lowest TABINDEX in the selected folder PAGE.

The dialog code can then detect which page was selected by using BEFORE INPUT, BEFORE DISPLAY or BEFORE CONSTRUCT of the corresponding sub-dialog block:

DIALOG ...
    INPUT BY NAME rec.* ...
       BEFORE INPUT -- focus goes to a field of this sub-dialog/folder page
           ...

If the focus is not in the context of the folder, the user can still select a folder tab to show up one of its pages, but the focus is not set to an element of that page: The focus remains in the current field, unless the user explicitly clicks or tabs to a field of the displayed page. This allows to view folder pages while entering data in another dialog, without leaving the current field, which would produce input validation and AFTER FIELD / AFTER INPUT blocks execution.