Created attachment 295938 [details] Working example. Cannot select both "xxx" and "foo = bar" in the attached example by clicking mouse cursor and dragging along line. Removing display: flexbox makes it possible, as expected.
Created attachment 295939 [details] Failing example
It's because RootInlineBox::selectionTopAdjustedForPrecedingBlock() assumes preceding block is always above the current line (while in flex layout, they could very well share the same line).
Created attachment 296322 [details] Patch
Comment on attachment 296322 [details] Patch r=me
Comment on attachment 296322 [details] Patch Clearing flags on attachment: 296322 Committed r209427: <http://trac.webkit.org/changeset/209427>
All reviewed patches have been landed. Closing bug.