Initial internal support for multiple webviews (#31417)

* Add multiple concurrent top-level browsing contexts

Co-authored-by: Delan Azabani <dazabani@igalia.com>

* Rename variables and comments

There are some variable and comments still use browser as names.
This commit renames them to webview.

* Update log message from web view to webview

* Revert offscreen_framebuffer_id rename

* Rename all web view to webview

* Cargo fmt

* Fix viewport/event/clear coordinates when multiview is disabled

* Only deprecate things when multiview is enabled

* Update WebViewManger with shown and invisible sets

Replace visible_webviews and native_window_is_visible with shown_webviews
and invisible_webviews. Add 4 more methods to set them accordingly. The
behavior of is_effectively_visible will return true if the wbview is in
shown_webviews set but not in invisible_webviews.

* Update variant behaviors

* Rename WebViewVisibilityChanged to MarkWebViewInvisible

* Fix unit test by marking id 3 visible again

* Update MarkWebViewInvisible and add UnmarkWebViewInvisible

* Update format and doc comments

* Clean up doc comments

* Address style and naming changes

* Rename UpdateWebView to UpdateFrameTreeForWebView

* constellation: send frame tree unconditionally over focus and feature

* Clarify shown and invisible sets in constellation WebViewManager

* Eliminate forward_to_constellation!()

* Actually remove the unused macro

* Don’t gate compositor changes on multiview feature flag

* Update todo in mouse event dispatch

* Pass all visible webview ids in a single ReadyToPresent message

* Fix compile and lint errors

* servoshell: fix gap between minibrowser toolbar and webview

* Fix failure in /_mozilla/mozilla/window_resizeTo.html

* Fix compile warnings

* Remove stray dbg!()

* Remove confusing “effectively visible” logic (see #31815, #31816)

* Allow embedder to show/hide/raise webviews without ipc

* Update root pipeline only when painting order actually changes

* Stop gating old focus and SetFrameTree behaviour behind Cargo feature

* Use webview_id and WebViewId in webview-related code

* Improve logging of webview-related embedder events

* Allow webview Show and Raise events to optionally hide all others

* Don’t do anything in response to WebViewPaintingOrder

* Remove WebViewPaintingOrder, since its payload is unreliable

* On MoveResizeWebView, only update root pipeline if rect changed

* Rename IOCompositor methods for clarity

* compositor: add event tracing; log webview ops even without ipc

* Add temporary debug logging

* Add more temporary debug logging

* Remove temporary logging in compositor

* Remove temporary debug logging

* Add temporary debug logging, but defer I/O until panic

* Capture a backtrace with each crash log entry

* Proper error handling without panicking in WebViewManager

* Clean up imports in constellation

---------

Co-authored-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

python/servo/testing_commands.py

@@ -162,6 +162,7 @@ class MachCommands(CommandBase):
             "crown",
             "constellation",
             "style_config",
+            "compositing",
         ]
         if not packages:
             packages = set(os.listdir(path.join(self.context.topdir, "tests", "unit"))) - set(['.DS_Store'])