OK, now this is interesting: Slickedit v24 does *not* have this problem. However, there's some details that matter: in both v24 and v25, the Find Symbols window (and, for comparison, others as well, such as activate-files-project) does not show as having the focus: the title bar remains without the "active" color. However, in v24 the windows still manages to capture keyboard input, whereas in v25, it does not capture keyboard input, until I click on the click to make it active.
Fixing either thing (the window really should pick up the focus, upon activation, and the keyboard input needs to be captured, upon activation) should solve this. What I mean is, getting the focus should normally cause the keyboard input to be captured, unless you've got special case code for keyboard input here. Anyway, I am just theorizing in ignorance of the actual implementation, of course.