This issue had been addressed in revision rev#80279 and will be available in version 6.5 .
Now, there is a context menu "Add Selection To" -> watch tools ..


About the fact that "dropping 'f' into the Watch window should not be allowed. The way it is now it just looks like a bug."

I partly agree, if the expression were created right away, I would agree, but how it is done, opening the new expression dialog, this behavior can be useful, in your case, drop the `f' ... then in the text field, you can prepend with "a."

When the `f' is a long feature name, this is appreciated.

So I close this issue. Feel free to reopen if you disagree.