A line filled with a gradient drew in its first stop's colour only. As
PowerPoint draws it, the gradient is laid out over the box the stroke
covers (the outline's bounds grown by half the line width, before the
line is cut back under its arrowheads), so a horizontal line's 90 degree
gradient runs across its width; arrowheads take the gradient where they
sit.
Wild synthetic set: gradient lines slide 8.43 -> 0.40; picture with a
gradient border 8.54 -> 7.67.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>