## QA PASSED See QA report document: 8589bd73-674b-44d1-a3f4-03bdc78f5d04 **Verification Summary:** - Code analysis confirms the fix correctly removes extra \`- UNIT_HEIGHT\` from Y calculations - Mathematical verification shows slot labels and devices now align correctly - All 111 frontend tests pass - No regressions found **Note:** Visual browser testing was blocked by a separate frontend authentication bug (infinite redirect loop on login). This is unrelated to APC-57 and should be tracked separately. Fix verified working.

Fixed the off-by-one error in rack slot visualization. **Root Cause:** The Y position calculation for devices had an extra `- UNIT_HEIGHT` that caused devices to render one slot higher than their actual position. **Files Changed:** 1. `frontend/src/components/draggable-rack-diagram.tsx` (lines 315-318, 711-714): - Fixed device Y position calculation from: `y = 20 + totalHeight - (device.rack_position + device.height_units - 1) * UNIT_HEIGHT - UNIT_HEIGHT` - To: `y = 20 + totalHeight - (device.rack_position + device.height_units - 1) * UNIT_HEIGHT` 2. `frontend/src/components/rack-diagram.tsx` (line 166-167): - Fixed slotY calculation with the same pattern **Math verification:** For a 1U device at rack_position=20 with totalUnits=42: - Before fix: y = 20 + 840 - (20+1-1)*20 - 20 = 440 (renders at slot 21) - After fix: y = 20 + 840 - (20+1-1)*20 = 460 (renders at slot 20, correctly) **Testing:** - All 111 frontend tests pass - Build succeeds without errors