Synthpod [1] ships a simple keyboard LV2 plugin you can drop anywhere in a strip for testing purposes (mouse-only for now, adding key handling support in the future is possible, though)
We donāt integrate a keyboard into the plugin GUI/editor window. There is a keyboard in every MIDI track in the editor, if you make the track a bit taller than the default. (It is called the āscroomerā because it comes with a slider that can be used to both zoom and scroll). That isnāt quite as integrated as putting it inside the plugin window. We may consider adding something like in a future version.
Others have noted that you can also connect an external virtual keyboard to Ardour. If you use current/recent versions of Ardour, its input will be automatically connected to the first selected MIDI track by default (and thus to the instrument plugin in that track, if there is one).
jack-keyboard is the solution that is most likely present already, āgrabbingā the keyboard does seldom work, so itās best to ākeep on topā the keyboard window and activate it again and again. I also like LMMSā interface(s) a lot for swift MIDI composing, I think itās a good example in that regard. But in Ardour, expanding the track vertically and clicking on the note keys also allows very simple testing.
What Iād love to see (or maybe eventually write myself, but Iām still lacking expertise) one day is a program that can isolate a cheap USB keybord from the nearest car boot sale, i.e. by its vendor & product id, and use it as a dedicated virtual MIDI keyboard rather than merging it with other input devices.
Iāve begun reading up on X and udev once when I wondered how feasible it was to have a US and a Spanish keyboard using their respective layouts side by side, but didnāt get anywhere with my current skill level, but yes, feasible it seemed.
thereās also the āMidi Sequencer 8x8ā(type ā8xā in the search plugin list), it loops itself while simulating the event of sending midi notes.