An "active" layout object is an object that currently has the focus. That object is very likely not a specific tab panel. If you have a cursor sitting in a field in you tab panel, for example, that field object, not the tab panel is the active layout object. If you press tab and a button highlights, the button object has the focus. I don't think you can give a tab control the focus, but even if you can, it's not likely to be 100% reliable for this purpose.
If you are using FileMaker 12, there's a nice set of new script triggers and get functions you can use to capture the object name of the currently selected tab panel.
If you are using an older version, you can use an OnObjectModify Trigger on the tab control to perform a script each time a tab panel is selected. That trigger can perform a rather laboriously constructed script that uses a series of IF-ElseIf steps with GetLayoutObjectAttribute to determine which panel is the font panel.