fix(overlay): flexible overlay with push not handling scroll offset and position locking (6.4.X)

[LeonardoGastonRossi]

This is a defect that was already fixed in #12624. But in my project we need this defect fixed in 6.4.X. I changed exactly the same files. The PR is exactly the same but targeting to 6.4.X.

• Fixes the position of flexible overlays with pushing enabled being thrown off once the user starts scrolling.
• Fixes flexible overlays with pushing not handling locked positioning. With these changes locked overlays will only be pushed when they're opened, whereas non-locked overlays will stay in the viewport, even when the user scrolls down.
• Fixes a potential issue due to a couple of variables being initialized together where one is set to zero and the other one is undefined.

Fixes #11365.

[LeonardoGastonRossi]

for some reason the CI is running against the lint rules from master.
Same happend in #13129

[LeonardoGastonRossi]

@crisbeto @jelbourn @andrewseguin guys, sorry, me again. Could you take a look to this when you have time, pls?

[jelbourn]

@crisbeto could you review this one?

[crisbeto]

@jelbourn this seems to be a cherry pick of my changes from #12624. The difference is that they're targeting the 6.4.x branch.

[jelbourn]

Yeah, I just wanted to make sure it was a clean cherrypick

[crisbeto]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "continuous-integration/travis-ci/pr" is failing
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[crisbeto]

Looks good @jelbourn.

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