How to restore horizontal application menu in KDE Plasma 5?
Operating System: Kubuntu 19.04
KDE Plasma Version: 5.15.5
KDE Frameworks Version: 5.57.0
Qt Version: 5.12.2
Kernel Version: 5.0.0-15-generic
OS Type: 64-bit
I've checked with Dolphin, Kate and Gwenview.
I think the problem is with the ☰ button in your title bar. Its presence suppresses the appearance of the horizontal menu.
Remove it by opening System Settings > Application Style > Window Decorations > Buttons tab and dragging the ☰ button off your title bar into the space below.
In Kate and Gwenview, the horizontal menu now can be toggled on/off with Ctrl+M.
In Dolphin, now Ctrl+M toggles between the horizontal menu and the Control (☰) button.
Edit
And it's the same with the Fedora 30 KDE Spin:
Operating System: Fedora 30
KDE Plasma Version: 5.15.5
KDE Frameworks Version: 5.58.0
Qt Version: 5.12.1
Kernel Version: 5.0.17-300.fc30.x86_64
OS Type: 64-bit