>>>Any ideas on what would cause the form close button to become disabled during normal data entry operation with a modeless form? The form seems to be otherwise fully functional, can change pages in pageframe, can select another form, can minimize the form, menu options are functional, etc.; just can't close it. Toolbar form close button is also disabled when that form is active. FWIW, some data entry on the page where this seems to occur is in a grid, but that remains fully functional as well.
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>>Check also message #925618
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>Thisform.Closable = .T. and that is never changed in code.

Try to trigger it back and forth, that's what this message suggests.