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

[crisbeto]

This is a resubmit of #11628.

• 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.

[jelbourn]

Caretaker note: this change has messed up menus in Firebase twice now. Do not merge without validating that the issue is resolved with a Firebase team member.

[ngbot]

Hi @crisbeto! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

[mmalerba]

@crisbeto figured out that's going on with the g3 failures. You added withLockedPosition to the menu. It seems like firebase has a button that triggers a menu and a screenshot test where they open that menu and then resize the screen. They're expecting the menu to reposition into a better place after the resize, but that is no longer happening. Probably what we need to do is lock the position during scrolling, but still potentially reposition when the window resizes

[crisbeto]

@mmalerba I'm not sure why somebody would have tests for this case, but it makes sense ¯\_(ツ)_/¯. I've reworked it so it treats repositioning after a page resize as a new render and it picks a new optimal position.

[davidmedina-contact]

Good work @crisbeto! Wondering when can this be merged? Anxiously waiting for a fix to this one

[lfdantoni]

Hi @crisbeto ! Good work, I am wondering if this PR was merged because I just saw that there is a new material version (6.4.7) and I can't find these changes applied. Thank you :)

[crisbeto]

It's in the 7.0 beta.

[LeonardoGastonRossi]

Hii @crisbeto how are you!? Is it possible for any chance that this fixed can be push in version 6?? Because It's a blocker for us, it's breaking the functionality in our site.

[amouly]

@crisbeto this issue breaks and existing functionality on our Production website. There is any chance we can push this code to a future 6.X.X release? If not, what's the blocker? There is any known workaround?

Thanks in advance.

[adamdport]

On Cordova, the menus sometimes don't appear to open at all because they're positioned off screen.

For those that end up here from google, it looks like this has been included in 7.0.0-beta-0.

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