When I open a file of a project, kate adds the project tab to the bar with "Documents" and "File Browser" on the left. It doesn't add the "in project" option to the global search&replace tab accessible via the button on the bottom of the window. Instead, I have to deactivate the project plugin and reactivate it in settings in order to get this option. I'm running Kate 16.12.3-1.1 from openSUSE Tumbleweed.
Should be fixed by https://phabricator.kde.org/D17296
Yep, should be solved.