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Objects Replaced by Navigations

There are 3 major types of navigations, each of which can cause different objects to be replaced. The general rules look something like this:

Class/Id in-process navigations cross-process navigations cross-group navigations
BrowserId 1 preserved preserved preserved
BrowsingContextWebProgress preserved preserved preserved
BrowsingContextGroup preserved preserved replaced
BrowsingContext preserved preserved replaced
nsFrameLoader preserved replaced replaced
RemoteBrowser preserved replaced replaced
Browser{Parent,Child} preserved replaced replaced
nsDocShell preserved replaced replaced
nsGlobalWindowOuter preserved replaced replaced
nsGlobalWindowInner replaced 2 replaced replaced
WindowContext replaced 2 replaced replaced
WindowGlobal{Parent,Child} replaced 2 replaced replaced
Document replaced replaced replaced

Types of Navigations

(in-process-navigations)=

in-process navigations

An in-process navigation is the traditional type of navigation, and the most common type of navigation when {ref}Fission is not enabled.

These navigations are used when no process switching or BrowsingContext replacement is required, which includes most navigations with Fission disabled, and most same site-origin navigations when Fission is enabled.

(cross-process-navigations)=

cross-process navigations

A cross-process navigation is used when a navigation requires a process switch to occur, and no BrowsingContext replacement is required. This is a common type of load when {ref}Fission is enabled, though it is also used for navigations to and from special URLs like file:// URIs when Fission is disabled.

These process changes are triggered by DocumentLoadListener when it determines that a process switch is required. See that class's documentation for more details.

(cross-group-navigations)=

cross-group navigations

A cross-group navigation is used when the navigation's response requires a browsing context group switch.

These types of switches may or may not cause the process to change, but will finish within a different BrowsingContextGroup than they started with. Like cross-process navigations, these navigations are triggered using the process switching logic in DocumentLoadListener.

As the parent of a content browsing context cannot change due to a navigation, only toplevel content browsing contexts can cross-group navigate. Navigations in chrome browsing contexts 3 or content subframes only experience either in-process or cross-process navigations.

As of the time of this writing, we currently trigger a cross-group navigation in the following circumstances, though this may change in the future:

  • If the Cross-Origin-Opener-Policy header is specified, and a mismatch is detected.
  • When switching processes between the parent process, and a content process.
  • When loading an extension document in a toplevel browsing context.
  • When navigating away from a preloaded about:newtab document.
  • When putting a BrowsingContext into BFCache for the session history in-parent BFCache implementation. This will happen on most toplevel navigations without opener relationships when the fission.bfcacheInParent pref is enabled.

State which needs to be saved over cross-group navigations on BrowsingContext instances is copied in the CanonicalBrowsingContext::ReplacedBy method.

Footnotes

  1. The BrowserId is a unique ID on each BrowsingContext object, obtained using GetBrowserId, not a class. This ID will persist even when a BrowsingContext is replaced (e.g. due to Cross-Origin-Opener-Policy). ↩

  2. When navigating from the initial about:blank document to a same-origin document, the same nsGlobalWindowInner, WindowContext and WindowGlobal{Parent,Child} may be used. This initial about:blank document is the one created when synchronously accessing a newly-created pop-up window from window.open, or a newly-created document in an <iframe>. ↩ ↩2 ↩3

  3. A chrome browsing context does not refer to pages with the system principal loaded in the content area such as about:preferences. Chrome browsing contexts are generally used as the root context in a chrome window, such where browser.xhtml is loaded for a browser window.

    All chrome browsing contexts exclusively load in the parent process and cannot process switch when navigating. ↩