Question:

I've specified a small offset (< 16px) to the bottom border of the browser window and it is fine in all browsers as long as there is no horizontal scrollbar.
However, when there is a scrollbar the results in various browsers differ. Depending on the offset value I specify, the layer may even be partly covered by the scrollbar (eg. in Netscape 4.x). Why is that?

Answer:

To be able to dynamically determine the layer position, the PLP script requests the available window width/heigth from the browser.
But browsers handle the presence or absence of scrollbars differently: some browsers subtract the height of the scrollbar from the window height when asked for the available window height, while others don't.

There is no way to explicitly detect the presence or absence of scrollbars, so the PLP script has no way of dynamically adjusting the position of a layer to that.

If you want to make certain that a layer is never partly covered by a possible scrollbar in any browser, then specify a minimum offset of about 16px to the bottom window border.