The generic function
pane-got-focus is called just before the focus is set by set-object-automatic-resize.
The supplied primary method does nothing. You may add methods on your own interface classes, which can be useful for example when the focus is set programmatically to a pane which is hidden inside a tab-layout or switchable-layout. Your method can check for this case and modify the layout as required.
CAPI User Guide and Reference Manual (Macintosh version) - 3 Aug 2017