Load the attached testcase and place the cursor anywhere except the very beginning or very end of the text. Using cmd + shift and the left/right arrow keys, create a selection. The selection will be everything from the cursor to the start or end of the content which is the same behavor as using the up/down arrows. This is a regression from shipping Safari which selects only to the start/end of that line.
Created attachment 12020 [details] Testcase
<rdar://problem/4960242>
This was fixed in r19417