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Final Analysis: Label Positioning Bug

The Root Cause

After extensive investigation, I've identified TWO CRITICAL MISMATCHES causing labels to appear on connection lines instead of box corners:

1. Inconsistent Sticky Header Activation

In label_renderer.go:

if e.diagram.Type == "sequence" && e.diagramScrollOffset > 7 {
    // Sticky headers are active

In tuieditor.go (actual rendering):

if e.diagram.Type == "sequence" && headerLines > 0 && startLine > headerLines {
    // We're scrolled past the headers - make them sticky

The label renderer uses a hardcoded value of 7, while the actual rendering uses dynamically calculated headerLines (which can vary based on the actual header content).

2. Wrong Y Coordinate Calculation for Sticky Headers

When sticky headers ARE active, the label calculation is:

viewportY = pos.Y + 1 + scrollIndicatorLines

With pos.Y = 2 and scrollIndicatorLines = 1, this gives viewportY = 4.

BUT, based on actual rendering output:

  • Line 1: Scroll indicator
  • Line 2: Empty space
  • Line 3: Empty space
  • Line 4: Box top border (╭──────╮) <- Where label SHOULD be
  • Line 5: Participant text (│ Client │) <- Where label APPEARS
  • Line 6: Box bottom border (╰──────╯)

The formula should be:

viewportY = pos.Y + 2 + scrollIndicatorLines

The Exact Problem

  1. Participants are positioned at Y=2 in the diagram coordinate system
  2. The box top border renders at Y=2 in diagram coordinates
  3. When converted to viewport coordinates with sticky headers:
    • Current calculation: 2 + 1 + 1 = 4 (points to line 4 in 0-indexed, but ANSI uses 1-indexed)
    • Actual need: 2 + 2 + 1 = 5 to account for the additional offset in sticky header rendering

Why This Happens

The rendering pipeline has multiple coordinate systems:

  1. Diagram coordinates: Where nodes are positioned (Y=2 for participants)
  2. Canvas coordinates: After applying scroll offset
  3. Viewport coordinates: After adding UI elements (scroll indicators, headers)
  4. ANSI coordinates: 1-indexed for terminal display

The label renderer is missing the correct transformation between these systems when sticky headers are active.

The Fix

In editor/label_renderer.go, line 49:

// CURRENT (WRONG):
viewportY = pos.Y + 1 + scrollIndicatorLines

// SHOULD BE:
viewportY = pos.Y + 2 + scrollIndicatorLines

AND check for sticky headers should match the actual rendering logic:

// Instead of hardcoded 7, use the actual header size calculation

Additional Issues Found

  1. Connection labels use separate logic from node labels, potentially causing inconsistencies
  2. Multiple coordinate transformation implementations exist across the codebase with subtle differences
  3. The header size calculation is duplicated in multiple places rather than being centralized

Verification

The test output clearly shows:

  • Box top border at line 4 (1-indexed)
  • Label calculation gives Y=4 (which in 1-indexed ANSI becomes line 4)
  • But the actual box corner is at line 4, not line 5

This off-by-one error is consistent across all scrolled scenarios with sticky headers.