artifact-canvas: full-row '+' affordance (GitHub-diff pattern) — structurally eliminate the hover travel-gap bug class

[amrmelsayed]

Origin

At pir-1237's dev-approval gate (2026-08-05), reviewer testing of the shipped #1236 fix (grace timers + pin-on-overlay-hover) surfaced residual UX gaps the damping approach cannot close:

• Hovering the right side of a wide block shows the '+' far away at the block's first line, left edge — a long diagonal travel even with grace.
• Dead strips (page padding, block margins) show nothing at all — no affordance is reachable from them.
• The gutter-overlay anchoring model (from vscode: markdown preview marker-aware features — inline REVIEW rendering + right-edge marker minimap #863) means the affordance's position is only loosely coupled to where the pointer actually is.

Proposal (reviewer's, endorsed at gate)

Adopt the GitHub-diff pattern: the whole row is the hover target, and the '+' renders inside the hovered block's own row (leading edge, vertically centered on the row under the pointer). This structurally eliminates the travel-gap bug class — there is no journey between trigger and target, so grace timers, pinning, and re-anchor suppression all become unnecessary and can be deleted.

Scope notes

• Supersedes: the artifact-canvas: '+' affordance disappears while mouse travels toward it; also undersized vs prose font #1236 grace+pin machinery (small, deliberate interim — shipped to fix the live bug immediately, per the option-(a) decision at the gate), the absolute-overlay anchoring contract from vscode: markdown preview marker-aware features — inline REVIEW rendering + right-edge marker minimap #863, and their tests.
• Must preserve: the composer flow (vscode: replace top-of-window Quick Pick input for adding review comments from the markdown preview with an inline composer #1107 inline-below portal), marker-card layout (vscode: markdown preview marker-aware features — inline REVIEW rendering + right-edge marker minimap #863's in-flow cards), keyboard activation parity (artifact-canvas: keyboard-first review navigation — jump keys, focus management audit, minimap focusability #1237's Enter/Space on focused block should light the same affordance), and the sizing/token fixes from artifact-canvas: '+' affordance disappears while mouse travels toward it; also undersized vs prose font #1236 (16px font token, 24px hit target) carried into the new placement.
• Design questions for the plan: hover target = block bounding box or full canvas row width (including padding strips)? What does the affordance do on blocks that already carry marker cards? Interaction with text selection (the '+' must not intercept drag-selection over the row).
• Test scenarios to inherit from the gate finding: right-edge hover, page-padding/margin strips, tall multi-line blocks, rapid vertical mouse travel.

PIR — supersedes approved layout decisions, design-sensitive. Related: #1236 (interim fix), #863 (current layout contract), #1107 (composer), #1237 (keyboard parity).