[LayoutNG] Initial README.md file.

third_party/WebKit/Source/core/layout/ng/README.md

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+# LayoutNG #
+
+This directory contains the implementation of blink's new layout engine

[Gleb Lanbin, 2016/10/10 23:07:55]

use Blink's instead of blink's?

[ikilpatrick, 2016/10/11 21:26:33]

Done.

+"LayoutNG".
+
+This README can be viewed in formatted form [here](https://chromium.googlesource.com/chromium/src/+/master/third_party/WebKit/Source/core/layout/ng/README.md).

[eae, 2016/10/10 23:17:05]

I know I suggested it but I'd be very reluctant to include a link that relies on
the current tooling and repository format. Given that these things have changes
at least four times in the last two years I doubt this url will last for more
than a year.

Sigh.

[ikilpatrick, 2016/10/11 21:26:33]

:( Yeah, I like the bug you filed w/ infra. This really needs to be exposed by a
maintained site.

+
+The original design document can be seen [here](https://docs.google.com/document/d/1uxbDh4uONFQOiGuiumlJBLGgO4KDWB8ZEkp7Rd47fw4/edit).

[eae, 2016/10/10 23:17:05]

This URL doesn't work.

[ikilpatrick, 2016/10/11 21:26:32]

did you copy the space in the review tool?

+
+## High level overview ##
+
+CSS has many different types of layout modes, controlled by the `display`
+property. (In addition to this specific HTML elements have custom layout modes
+as well). For each different type of layout, we have a
+[NGLayoutAlgorithm](ng_layout_algorithm.h).
+
+The input to an [NGLayoutAlgorithm](ng_layout_algorithm.h) is the same tuple
+for every kind of layout:
+
+ - The [NGBox](ng_box.h) which we are currently performing layout for - the

[Gleb Lanbin, 2016/10/10 23:07:55]

should "- the following" start from a new line?

[ikilpatrick, 2016/10/11 21:26:33]

Acknowledged. Changed to sentence.

+   following information is accessed:
+
+   - The [ComputedStyle](../../style/ComputedStyle.h) for the node which we are
+     currently performing laying for.
+
+   - The list of children [NGBox](ng_box.h)es to perform layout upon, and their
+     respective style objects.
+
+ - The [NGConstraintSpace](ng_constraint_space.h) which represents the "space"
+   in which the current layout should produce a
+   [NGPhysicalFragment](ng_physical_fragment.h).
+
+ - TODO(layout-ng): BreakTokens should go here once implemented.
+
+The current layout should not access any information outside this set, this
+will break invariants in the system. (As a concrete example we intend to cache
+[NGPhysicalFragment](ng_physical_fragment.h)s based on this set, accessing
+additional information outside this set will break caching behaviour).
+
+### The Box Tree ###
+
+TODO(layout-ng): Document with lots of pretty pictures.

[Gleb Lanbin, 2016/10/10 23:07:55]

TODO should be followed by username, bug ID or email. layout-ng is not
associated with anything.

[ikilpatrick, 2016/10/11 21:26:33]

Done.

+
+### The Fragment Tree ###
+
+TODO(layout-ng): Document with lots of pretty pictures.
+
+### Contstraint Spaces ###

[Gleb Lanbin, 2016/10/10 23:07:55]

typo. Contstraint -> Constraint

.nit should we drop "The" in titles to keep it simple?

[ikilpatrick, 2016/10/11 21:26:33]

Done.

+
+TODO(layout-ng): Document with lots of pretty pictures.
+
+## The Block Flow Algorithm ##
+
+This section contains details specific to the
+[NGBlockLayoutAlgorithm](ng_block_layout_algorithm.h).
+
+### Margin Collapsing ###

[Gleb Lanbin, 2016/10/10 23:07:55]

it's named Collapsing Margins in
https://www.w3.org/TR/CSS2/box.html#collapsing-margins

[ikilpatrick, 2016/10/11 21:26:33]

Done.

+
+TODO(layout-ng): Document with lots of pretty pictures.
+
+### Float Positioning ###
+
+TODO(layout-ng): Document with lots of pretty pictures.