The problem happens only building for GTK3; building the exact same code for GTK2 works fine.
In the code, posted to https://stackoverflow.com/questions/54529359/gtkmm-3-text-view-anchored-in-text-view-cannot-get-cursor-inside , I'm having a text view anchored into another text view. The problem is that even if I mouse click, I cannot have the cursor move inside the nested text view to write text. The cursor can instead easily get into the anchored text entry just below.