Make it possible to draw layers outside of the window

[paulrouget]

Why: we want to be able to draw a transparent toolbar on top of content, but the content covered by the toolbar should not be within the window. So we want to be able to tell WR to not clip at the window boundaries but a larger rectangle.

I tried to prototype this: master...paulrouget:outOfWindowRendering

But it's not working so well. Only the background of the page is drawn outside of the window boundaries.

In this screenshot, the window starts at black line. After scrolling:

[glennw]

I pushed a very hacky proof of concept branch, that (I think) does what you're looking for - https://github.com/glennw/webrender/tree/overlay-demo.

With the following branch, when you first load a page, the content is 100px below the top of the window:

Once you start to scroll, the content scrolls up to the top of the window, which would allow you to place your transparent toolbar on top of the content:

Feel free to have a play around with that branch - and once we get something that does what you're looking for, we can work out the right API.

[paulrouget]

This is working very well!

Two issues:

• the scrollbar should be shifted as well
• position:fixed;bottom:0 is off screen

As for the API, ideally we could do something like that: api.set_clipping_region(Rect)

[paulrouget]

It would help me to get that in master. I need a way to update the clipping region dynamically.

Maybe via: Browser::handle_event(WindowEvent::UpdateClippingRegion(Rect)).

[paulrouget]

PR wip: #951

[glennw]

This has landed in WR in #951. I'll do a WR update to Servo today. We can then open more specific bugs in Servo related to the public API.