The selection for multi-column layout breaks down when in the gaps or blank spots of the block. positionForPoint has code to adjust the point to the correct column for the old multi-column code, but it has not been patched yet for the new multi-column code.
Created attachment 229498 [details]
Attachment 229498 [details] did not pass style-queue:
ERROR: Source/WebCore/rendering/RenderMultiColumnSet.cpp:889: Weird number of spaces at line-start. Are you using a 4-space indent? [whitespace/indent] 
ERROR: Source/WebCore/rendering/RenderMultiColumnSet.cpp:918: Should have only a single space after a punctuation in a comment. [whitespace/comments] 
ERROR: Source/WebCore/rendering/RenderMultiColumnSet.cpp:954: Should have only a single space after a punctuation in a comment. [whitespace/comments] 
Total errors found: 3 in 6 files
If any of these errors are false positives, please file a bug against check-webkit-style.
Comment on attachment 229498 [details]
Looks good to me.
Landed in r167404.