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.

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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