feat(menu): add input to add overlay pane classes

[andrewseguin]

Required for GMDC, which needs to automatically add a CSS class to the overlay menu using injected options

[crisbeto]

We have some code on MatMenu that proxies the classes from the mat-menu node to the overlay. Couldn't we take advantage of that?

[andrewseguin]

The current panel class input doesn't work because of how we are implementing GMDC. In short, users will import a module GmatMenuModule that import/exports the menu directives. We need to be able to set some custom classname with the module, so this change lets me provide it through the options provider.

[crisbeto]

I see. I think in this case it would make more sense to just use the injection token (as you're doing already) without exposing it as an input. Having it be an input means that we'd also have to ensure that the value on the overlay stays in sync with the input value.

[andrewseguin]

I believe they are distinct - the class input currently on the mat-menu will be applied to the element with CSS class mat-menu-panel

The overlayPanelClass will be applied to what the overlay considered its pane, which contains the aforementioned mat-menu-panel.

<div class="cdk-overlay-container">
  <div class="cdk-overlay-connected-position-bounding-box">
    <div class="cdk-overlay-pane"> <-- new input
      <div class="mat-menu-panel"> <-- existing input

I considered making the injected option affect the existing input but I didn't want to have to deal with figuring out how to make sure we merge the options and input together.

[crisbeto]

Sorry, what I meant is that we should do what you're currently doing, but we shouldn't add a new input. IMO the only place the panelClass would be configured would be the injection token.

[andrewseguin]

Probably would make sense to incrementally add anyways. This is a very niche edge case and would be confusing trying to explain the different between the inputs to users. I removed the input and kept the rest

[crisbeto]

LGTM

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