The old code didn't handle the mouse click (no good), it allowed for this behavior, but also would cause things like clicks passing through buttons and activating stuff in the viewport (no good). So - the SButton no longer returns Unhandled, and returns handled. The correct way to do this now would be to override OnPreviewMouseDown, instead of OnMouseDown - and then have the same code as before.
When using the left mouse button to click and drag a button, the on drag detected will not fire even when using precise click. This was not the case in previous versions of the engine (Works as expected in 4.16, issue appears in 4.17.1)
Result: The on drag detected does not fire off when using precise click
Expected: When using precise click the button would not consume the mouse input and the on drag detected would fire off when clicking and dragging
Head over to the existingAnswerHub thread and let us know what's up.