when is a field clearable?

[Jerome Yuzyk]

The "clear this field" icon that appears on many fields in KDE apps and 
things browsed with Konqueror is really handy for mouse-based navigation. 
However, not all fields get this option - most notably the Google entry field 
(on www.google.*) doesn't get one, whereas fields on the CPAN site do.

What determines whether a field gets one of these? Does Google do something 
different with their field?
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