You should set the widget to which your popover is relative to, by using
Gtk.Popover.relative_to; then you can use
Gtk.Popover.pointing_to and the rectangle with the location of the cursor. To get that, use
Gtk.Entry.cursor_position to retrieve the index of the cursor in characters and transform it into a location within the Pango.Layout of the
Gtk.Entry widget; if the entry is allowed to scroll, you should also get the
Gtk.Entry.scroll_offset and subtract it from the coordinates returned by Pango.
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