I just found out what is wrong with my Delphi IDE Explorer in Delphi 10.3 Rio. The symptom is that the Follow Focus option and the Select Active button no longer work in some forms, in particular in the options dialog (Tools -> Options / Run -> Parameters / Project -> Options). The reason is that this dialog now uses the Screen.OnActiveControlChange event itself and does not bother to call the original event. In previous Delphi versions it did not use this event. Delphi IDE Explorer hooks it to track the control that has the focus and provide the functionality mentioned above. It does call the original event though.
Interestingly the IDE restores the previous event when closing the options dialog so Delphi IDE Explorer gets back its functionality for other dialogs. Unfortunately that does not apply to the dialogs called from within the Options dialog.
Nobody at Embarcadero seems to have read my proposal about Safe event hooking for Delphi IDE plugins or maybe they chose to ignore it. (I guess the number of people who have read it can be counted on the fingers of one hand anyway 😉 )
I have to investigate whether there is any safe method to get my plugin work again. Until then, you will have to live without the Follow Focus feature.