incremental search abandoned on mouse movements

[Mick Bert]

I am using
GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
of 2018-09-18
on a RedHat machine through Xvnc TigerVNC 1.8.0, and RealVNC viewer 5.0.1
on windows 10

Since few days I've been observing a strange behavior: I start an
incremental search (C-s), and emacs hilights all the visible matches, but
as soon as I move the mouse pointer, the search is cancelled (just like I
pressed C-g).
Have I activated unconsciously a kind of option or function?
Have anyone experienced something similar?

-- 
Mick