After installing Noto Sans CJK (super OTC) or CJK OTC (regular weight) at http://www.google.com/get/noto/help/cjk/, 

the following minimal test can be used:

data:text/html; charset=utf-8, <span style="font-family:Noto Sans CJK KR;">한〮</span>

The rendering of the above minimal test. Note that in the omnibox, U+302E (single dot centered vertically) is rendered correctly (to the left of '한'), but in the contents area, it's rendered with a dotted circle as if there's no base character. 

Deleted: Screen Shot 2016-03-14 at 오후 12.50.28.png 108 KB

	Screen Shot 2016-03-14 at 오후 12.50.28.png 108 KB View Download

> in the omnibox, U+302E (single dot centered vertically) is rendered correctly

Well, it's not exactly correct. The spacing is too tight. It uses the Mac OS default font for '한' and only U+302E may have come from Noto Sans CJK

Thanks for the report and test case, we have issues with fallback for combining characters that follow the base, I'll mark this a duplicate of 591346.

> Additional Information: This is a regression. It used to work fine. Firefox does not have this problem. 

It may have looked acceptable, but I tend to think we incorrectly combined the tone mark and the base from different fonts, compare  issue 504745 .

> It may have looked acceptable, but I tend to think we incorrectly combined the 
> tone mark and the base from different fonts, compare  issue 504745 .

Hmm.. could that happen even with an explicit font specified (as in a test given in comment 1)?  How about cases where the only font covering U+302E and U+302F is 'Noto Sans CJK' (when an explicit font is not specified) ?  

True, the case in comment #1 is definitely different, I am unmerging this from the fallback issue.  Suspecting an issue with how we handled collections, I tried with the Noto Sans CJK .ttc as well as with the Korean only, single typeface font. But the issue happens in both cases. hb-shape shapes the "한〮" sequence correctly with these fonts. 

The collection should not be an issue. It should be 'transparent' to Blink. 

Believe me :-) This used to work fine before. I produced a sample image to include in Google Korea Blog post using Chrome multiple times :-)

https://ko.wikisource.org/wiki/%ED%9B%88%EB%AF%BC%EC%A0%95%EC%9D%8C%EC%96%B8%ED%95%B4 is a page I always use to check 'Old Hangul' support (it explicitly specifies Noto Sans CJK KR in their CSS and it used to work fine). 

Perhaps, we should add a layout test using Noto Sans CJK (or a small subset thereof)

The shaper code correctly handles this but the CJK optimized segmentation in CachingWordShapeIterator breaks it. We need to exclude the combining marks when segmenting.

Kojii, could you take a look? You wrote much of the CJK code path in CachingWordShapeIterator.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a7051b4065ce7dabe0e1ab758396f7e5c770d02d

commit a7051b4065ce7dabe0e1ab758396f7e5c770d02d
Author: kojii <kojii@chromium.org>
Date: Wed Apr 06 16:29:03 2016

Fix Hangul Tone Marks not to delimit in CachingWordShapeIterator

This patch includes two fixes for CachingWordShapeIterator not to
delimit Hangul Tone Marks from base characters.

The first fix is not to delmit non-CJK runs when it encounters CJK
marks.

The second fix is to exclude:
  U+302E HANGUL SINGLE DOT TONE MARK
  U+302F HANGUL DOUBLE DOT TONE MARK
from the isCJKIdeographOrSymbol set. This change makes the Hangul tone
marks consistent with other Hangul characters.

BUG= 594658 

Review URL: https://codereview.chromium.org/1861033003

Cr-Commit-Position: refs/heads/master@{#385475}

[modify] https://crrev.com/a7051b4065ce7dabe0e1ab758396f7e5c770d02d/third_party/WebKit/Source/platform/fonts/Character.h
[modify] https://crrev.com/a7051b4065ce7dabe0e1ab758396f7e5c770d02d/third_party/WebKit/Source/platform/fonts/CharacterPropertyData.cpp
[modify] https://crrev.com/a7051b4065ce7dabe0e1ab758396f7e5c770d02d/third_party/WebKit/Source/platform/fonts/CharacterPropertyDataGenerator.h
[modify] https://crrev.com/a7051b4065ce7dabe0e1ab758396f7e5c770d02d/third_party/WebKit/Source/platform/fonts/CharacterTest.cpp
[modify] https://crrev.com/a7051b4065ce7dabe0e1ab758396f7e5c770d02d/third_party/WebKit/Source/platform/fonts/shaping/CachingWordShapeIterator.h
[modify] https://crrev.com/a7051b4065ce7dabe0e1ab758396f7e5c770d02d/third_party/WebKit/Source/platform/fonts/shaping/CachingWordShaperTest.cpp