[WIP] Split out Surface from Window

[jgcodes2020]

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

This PR splits some methods of Window into a new supertrait Surface, laying groundwork for the implementation of #3928.

As stated there, this split is needed because popups and subsurfaces may not necessarily allow the same operations as windows.

[kchibisov]

Still wonder whether to use View/Subview for that instead of Surface/Subsurface, since the Subsurface doesn't make much sense for people not familiar with Wayland.

Also not sure about the set_cursor API and it's not clear how Ids should generally work here. Like our WindowId should become some kind of Surface/ViewId and then when it comes to events surface/window/desktop is not well established throughout the API, so I'd probably hold a bit on that.

[kchibisov]

I'd also add a as_window(&self) -> Option<&dyn Window> on Surface trait to help with upcasting when passing just Surface around and raw-window-handle should be for Surface.

[jgcodes2020]

Still wonder whether to use View/Subview for that instead of Surface/Subsurface, since the Subsurface doesn't make much sense for people not familiar with Wayland.

I'll argue that Surface is used by graphics APIs and Wayland. I don't want this to devolve into bikeshedding, so I'll leave it as Surface unless the maintainers collectively agree that View is the better naming convention.

Also not sure about the set_cursor API

The set_cursor API is copied verbatim from the original window trait. It should be possible to have cursors per NSView; if not, that can be emulated by changing the cursor on PointerMoved events.

and it's not clear how Ids should generally work here. Like our WindowId should become some kind of Surface/ViewId and then when it comes to events surface/window/desktop is not well established throughout the API, so I'd probably hold a bit on that.

I already addressed event handling in my proposal. Summary:

• I agree with WindowId becoming SurfaceId.
• Pointer events can be received by any surface (window, popup, or subsurface), but keyboard events can only be received by windows.
• Events aren't bubbled. Not every platform does this, and the platforms that do should be able to stop them from being bubbled.

[jgcodes2020]

With the Windows commit, that's every platform I can test on my machine at the moment. I don't have a Mac nor do I have Android development set up at the moment.

[MarijnS95]

Now that I'm assigned for review: I'd like this PR description and the resulting squashed commit to contain a description and justification of the respective changes, rather than merely mentioning that it addresses some part of a large discussion thread in an issue.

That should make it easier to review and backtrack.

[jgcodes2020]

Now that I'm assigned for review: I'd like this PR description and the resulting squashed commit to contain a description and justification of the respective changes, rather than merely mentioning that it addresses some part of a large discussion thread in an issue.

That should make it easier to review and backtrack.

I've updated the PR description.