In some cases its seems, after selecting an item (for instance a container or input element) on the realtime canvas, impossible to use the right-click context menu and select "delete".
A number of options is grayed out (including "delete").

It seems that when clicking on the right mouse button focus on the element is lost.

Any explanation for this behaviour?

Usually it's not a problem and it works as expected, sometimes it doesn't.

If you find that happening, try re-opening the project file and that should correct any display issues like that.

There are some 'minor' things like that happening, and closing the programme and reopen seems to be the solution. But why do they happen at all?
Some other such things: Selecting px instead of rem on dimensions, margins or padding doesn't make the value change, happens more often than not. I either have to close and restart the programme, or select something else first, % or em etc. before selecting px.
Someone else recently reported that the slider was gone from the app. I've also had that happen. Close and restart fixes it.
And these things happen in all three of those responsive apps.

Thank you for acting so fast.
Yes, while you answered my question, I found out myself that saving and restarting solved the problem.

I agree with Inger that these kind of "problems" spoil your otherwise great apps.
It looks like the apps act a little bit nervous on the mouse. Double clicking - by accident - sometimes moves elements to a different place (out of their container).
It would be much better if we could also select and delete elements in the inspector pane (yes, it has been asked by others as well).

Inspector - Select - Main Menu - Edit - Delete

Select object, right-click, select action is more logical IMO