When using a QPlatformInputContext based virtual keyboard, I noticed that the keyboard only shows natively when the input widget is clicked. If either a setFocus call is used, or a tab is used to change the focus the virtual keyboard does not show. In the setFocus case, directly calling InputMethod->show did not help as while showInputPanel was invoked, the setFocusObject call had not yet occured (and in our case, that was needed for proper virtual keyboard operation).

I managed a workaround by subclassing the input widget and adding a timer to call show after enough time (about 50 ms) for the setFocusObject call to have occurred. 

What I think is needed here is more consistent operation of the QPlatformInputContext for the different types of focus changes. This includes both informing the context of the current focus object and calling set panel.