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# What is RenderDocument?
## TL;DR
Chrome currently switches to a new RenderFrameHost
when loading a new document
if the render process is different to the previous one.
The RenderDocument project is about making the switch to happen unconditionally.
This:
* Eliminates the logic for navigating inside the same RenderFrameHost
* Makes RenderFrameHost in the browser process 1:1 with the Document.
* Prevents security bugs,
e.g. reusing the data/capabilities from the wrong document.
## Details
Previously when we navigate a frame from one page to another,
the second page may appear in a new RenderFrame
or we may reuse the existing RenderFrame to load the second page.
Which happens depends on many things,
including which site-isolation policy we are following
and whether the pages are from the same site or not.
With RenderDocument,
the second page will always use a new RenderFrame
(excluding navigation within a document).
Also when reloading a crashed frame
we reused the browser-side RenderFrameHost.
With RenderDocument we create a new RenderFrameHost
for crashed frames.
## Read more
https://crbug.com/936696
[design doc](https://docs.google.com/document/d/1C2VKkFRSc0kdmqjKan1G4NlNlxWZqE4Wam41FNMgnmA)
[high-level view of the work needed](https://docs.google.com/document/d/1UzVOmTj2IJ0ecz7CZicTK6ow2rr9wgLTGfY5hjyLmT4)
[discussion of how we can land it safely](https://docs.google.com/document/d/1ZHWWEYT1L5Zgh2lpC7DHXXZjKcptI877KKOqjqxE2Ns)
# Stages
We have 3 stages that are behind flags.
1. crashed-frames:
A new `RenderFrameHost` is used for reloading a crashed document.
2. subframes:
A new `RenderFrameHost` is used for every nested document.
3. main frames:
A new `RenderFrameHost` is used for every document.
# Test changes
## RenderFrameHost reference becomes invalid
Enabling this for subframes and main frames causes many tests to fail.
It is common for tests to get a reference to a RenderFrameHost
and then navigate that frame,
assuming that the reference will remain valid.
This assumption is no longer valid.
The test needs to get a reference to the new RenderFrameHost,
e.g. by traversing the frame tree again.
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