This happens on Solaris with jdk1.1.6 and swing 1.0.2.
Does not happen under Windows

1.) Compile and start program SplitPaneTest from Attachments

You will see a window with a splitpane that has as right component another splitpane, a textfield and two buttons.

2.) Click inside one of the text areas in the right splitpane to give it focus.
3.) Resize window

Observe: text area loses focus, clicking anywhere in window doesn't make anything regain focus (you have to click outside to another window, click back in and here click on another component and not on the previous focus owner)

Second experiment:

1.) Start SplitPaneTest again
2.) Give one of the text areas on the right splitpane focus
3.) Move vertical divider

Observe: same thing, focus is lost and clicking anywhere doesn't regain it

These two experiments show that whenever a component inside a splitpane has focus and the splitpane is resized (in a certain way, see experiment 3) the focus is lost and very hard to regain.

Experiment 3:

1.) Start SplitPaneTest
2.) Put focus in one of the text areas of the right splitpane and hit the resize splitpane button.

Observe: it loses focus but you can regain it by clicking in it again