mat-menu: xPosition and yPosition binding not working

[neetu-pandey]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

xPosition and yPosition binding should work

What is the current behavior?

The binding does not work right now

What are the steps to reproduce?

Providing a StackBlitz reproduction is the best way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

https://stackblitz.com/edit/angular-7dtixc

What is the use-case or motivation for changing an existing behavior?

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Material v6.0

Is there anything else we should know?

#4213 mentions that the issue is fixed, but it is still reproducible.
The initial value of the variable is taken, subsequent change to the variable for positionX and positionY is not considered

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