Cannot get button press event on drawing area

[Радомир Хаџић <radomirhadzic46 gmail com>]

I want to get mouse movement events when a user moves cursor while
holding button 1 but am unable to.
Here, I wrote a small piece of code that illustrates the problem. I've
sent it as an attachment but you can also see it here:
https://pastebin.com/WcZrtBQR.

Attachment: main.cpp
Description: Text Data