See PHI1834. It's not obvious why this "+1" is present in the code, but it causes inlines to be adjusted incorrectly when a file is not modified across changes. See D21435.

Remove it, which appears to produce accurate adjustment behavior.

• See D21435 for instructions to build a change, where a file with lines "A-Z" is unmodified across Diff 1 and Diff 2.
• Left inlines on lines 14, 17-19, and 16-26 (end of the file) on Diff 1.
• Before: saw inlines incorrectly adjusted to lines 15, 18, and 17 on Diff 2. Before D21435, the last inline was culled by the rendering engine.
• After: saw inlines correctly adjusted to lines 14, 17, and 16 (the same lines as the original), render properly, and highlight the correct lines when hovered.