I walk through the container's Resize code in the debugger, all works as advertised.

If I run the code without the debugger, the TreeView doesn't resize as it should until I click
somewhere in the control.

Hmmmm - this one smells like a timing issue to me.

A strategically placed DOEVENTS in your code should solve the problem, but without seeing your code, it is difficult to hazard a guess as to where it should go.

Some other things to check are the setting of _VFP.autoyield and SYS( 2333 )