fix(canvas): selected reverse line no longer hides its difference-blend#51
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Previously the selection overlay rendered as a solid line directly on top of the body. For reverse (^LRY) lines that meant the selection covered the inversion visualisation — text or shapes layered under the line stopped showing through as long as it was selected. Switch to an Illustrator-style outline pattern: two thin parallel selection-coloured strokes offset perpendicular to the body, so the body's difference-blend keeps painting the underlying content while the outline marks the selection alongside it. Same render path for reverse and non-reverse lines — no special case needed. Offset = strokeWidth / 2 + 1 (1 px clearance outside the body). Guard against zero-length lines (would div-by-zero in the perpendicular unit-vector calc).
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This pull request updates the selection rendering for LineObject by replacing the single selection stroke with two parallel outlines offset perpendicularly from the line body. This change preserves the line's blend mode and provides a clearer selection affordance. The review feedback suggests improving code readability by extracting repeated coordinate calculations into intermediate variables and refining the offset logic to ensure the intended visual gap between the line and its selection outline.
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Reviewer-spotted (gemini-code-assist on PR #51): - Hoist dispX2-dispX1 / dispY2-dispY1 into dx, dy locals; less repetition in the perpendicular-vector math. - Offset was lineStrokeWidth / 2 + 1, which puts the selection stroke *centre* 1 px past the body's edge. With a 1 px-wide selection stroke that leaves an actual visual gap of only 0.5 px between the edges. Bump to +1.5 so the gap matches the comment.
Reviewer-spotted (gemini-code-assist on PR #51): - Hoist dispX2-dispX1 / dispY2-dispY1 into dx, dy locals; less repetition in the perpendicular-vector math. - Offset was lineStrokeWidth / 2 + 1, which puts the selection stroke *centre* 1 px past the body's edge. With a 1 px-wide selection stroke that leaves an actual visual gap of only 0.5 px between the edges. Bump to +1.5 so the gap matches the comment.
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Previously the selection overlay rendered as a solid line directly on top of the body. For reverse (^LRY) lines that meant the selection covered the inversion visualisation — text or shapes layered under the line stopped showing through as long as it was selected.
Switch to an Illustrator-style outline pattern: two thin parallel selection-coloured strokes offset perpendicular to the body, so the body's difference-blend keeps painting the underlying content while the outline marks the selection alongside it. Same render path for reverse and non-reverse lines — no special case needed.
Offset = strokeWidth / 2 + 1 (1 px clearance outside the body). Guard against zero-length lines (would div-by-zero in the perpendicular unit-vector calc).