[Lazarus] TPageControl suddenly stopped working

[Tail Kinker]

I am working on a project that has two TPageControl instances.  One is 
embedded inside a TTabSheet of another.  It is the embedded one that for 
whatever stupid reason has stopped working.

This behaviour started after I set one of the TTabSheets in the 
TPageControl disabled, but did not cease after I re-set the TTabSheet 
back to enabled.

Can anyone think what might be causing this behaviour?