Falkon doesn't support triggering right click via long press on touchscreen, which makes it awkward to use in touchscreens. It works in kmail, akregator and simplebrowser, so webengine definitely supports it. It would be nice if it worked in falkon too.
FWIW, I tried Falkon on Kubuntu 18.04 (Falkon 3.0.0 with WebEngine 5.9.5) and a Cosmic daily build (3.0.1; same WebEngine) on a laptop with a touchscreen, and I was getting the context menu with a long press. What version of Falkon and WebEngine are you trying?
falkon 3.0.1, webengine 5.11.0. Also tested 5.10.1, and still doesn't work
Do you have mouse gestures plugin enabled?
(In reply to David Rosca from comment #3) > Do you have mouse gestures plugin enabled? Ah yes, that was it. Disabling it makes it work.