The area to enter or edit a display filter string,
see Section 6.4, “Building display filter expressions”
. A syntax check of your filter string is done while you are typing.
The background will turn red if you enter an incomplete or invalid
string, and will become green when you enter a valid string. You can
click on the pull down arrow to select a previously-entered filter
string from a list. The entries in the pull down list will remain
available even after a program restart.
After you've changed something in this field, don't forget to press
the Apply button (or the Enter/Return key), to apply this filter
string to the display.
This field is also where the current filter in effect is displayed.