Anchor pull request comments to content to which they relate, not to a fixed line number

EXPECT: When I create a pull request comment, I expect this comment to remain anchored to the content to which the comment relates.
ACTUAL: If changes get merged into the "master" branch underneath the PR that cause the related content to move, the comment will no longer be attached to the original content. The anchoring appears to specify a fixed line number, instead of tracking with the original content from revision to revision.