Precise Click is not working correctly with Buttons

A licensee reported an issue where a recent change to SButton.cpp in 4.13 causes an issue with Precise Click in multiple situations, his example being the one described in the repro steps, which is an often use on Mobile apps.

I couldn't find the source of the change as I don't have access to the stream it was changed in, but you can see the change in perforce's diff checking when comparing SButton.cpp at CL 3033489 in Dev-Main to its previous revision.

Regression?: Yes
This did not happen in 4.12.5 release (//UE4/Release-4.12/ CL#3039270)
The suspected change that caused this was merged into //UE4/Dev-Main/ CL#3033489
This does occur in 4.13.0 release (//UE4/Release-4.13/ CL#3106830)

1. Download the attached project and open it
2. Press Play to start PIE
3. Click a button (Left-click) and hold it and then click to try to drag and scroll (Right-click)
4. After scrolling some, let both clicks go.
   Result: Sometimes, the button your cursor is over at the end will be triggered (Printing "Hello" in this case). The scrolling is also irregular as well.
   Expected: The button should never be triggered by this action as it is set to Precise Click and the scrolling should be consistent throughout