1. Breeze theme selected. 2. Started Kdenlive, all title bars visible for project, effect & monitor windows. 3. At some point (probably whilst Kdenlive was running, but not sure about this) theme changed to Breeze Dark. 4. At some point after that I noticed the title bars were missing from the windows above. 5. Checking and un-checking View > Show title bars has no effect.
It appears the title bars are not visit when panels are stacked within Kdenlive and accessed via tabs. Un-stacking the panels enables them to be moved around using the panel's title bar. Although not a bug, this is not intuitive.