On my system (Fedora 24 x64 with yumex-dnf 4.3.3), hitting Ctrl-F to show the search bar does not work. The packages list panel very briefly slides down a very short distance, as if the search bar were about to come into view, but then the packages list settles back into place and the search bar never renders. I can only show the search bar by clicking the magnifying glass button in the top right.

Interestingly, Ctrl-F will hide the search bar if it's already being displayed.

I started yumex-dnf in debug mode. Waited for it to check to metadata updates. Hit Ctrl-F a few times. Clicked on the search button. Then closed yumex. (I don't think it generated any useful output, though.)

$ yumex-dnf --debug
00:06:34: looking for /home/terrycloth/.themes/Adwaita/apps/yumex.css
00:06:34: looking for /usr/share/themes/Adwaita/apps/yumex.css
00:06:34: theme color : #000000
00:06:35: dnfdaemon api version (2)
00:06:35: Lock the DNF root daemon
00:06:41: installonly_limit = 5
00:06:41: clean_requirements_on_remove = True
00:06:41: get-packages : updates 
00:06:42: get_packages took 1.72 sec
00:06:42: get-packages : updates 
00:06:42: get_packages took 0.00 sec
00:06:42: get-packages : obsoletes 
00:06:43: get_packages took 0.02 sec
00:06:43: populate took 0.00 sec
00:06:43: get-packages : updates 
00:06:43: get_packages took 0.00 sec
00:06:43: get-packages : updates 
00:06:43: get_packages took 0.00 sec
00:06:43: get-packages : obsoletes 
00:06:43: get_packages took 0.00 sec
00:06:43: populate took 0.00 sec
00:07:03: Unlock the DNF root daemon
00:07:03: Exit the DNF root daemon
00:07:03: Saving config on exit