Fix drag-and-drop hover detection and slot targeting

Two root-cause bugs fixed:

1. Port <button> elements inside ModuleBlock had pointer-events:auto (browser
   default), so document.elementFromPoint() hit them instead of the RackSlot
   behind them whenever the cursor was over an occupied slot. Fixed by toggling
   body.rack-dragging during any drag, which applies a CSS rule that forces
   pointer-events:none !important on .module-block and all descendants.

2. onDragMove pointer-position reconstruction (activatorEvent.clientX + delta.x)
   was slightly off because delta is measured from the initial mousedown, not
   the activation point. Replaced with a native window pointermove listener
   (capture phase) that gives exact clientX/Y — no reconstruction needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/components/rack/RackPlanner.tsx

@@ -8,7 +8,6 @@ import {
   useSensors,
   type DragStartEvent,
   type DragEndEvent,
-  type DragMoveEvent,
 } from '@dnd-kit/core';
 import { SortableContext, horizontalListSortingStrategy, arrayMove } from '@dnd-kit/sortable';
 import { toast } from 'sonner';
@@ -61,13 +60,10 @@ function ModuleDragOverlay({ label }: { label: string }) {
  * Resolve which rack slot (if any) is under the pointer during a drag.
  *
  * Strategy: elementFromPoint at the current pointer coordinates.
- * - ModuleBlock has pointer-events:none when isDragging, so it is transparent.
+ * - All ModuleBlocks get pointer-events:none via the body.rack-dragging CSS rule,
+ *   so elementFromPoint sees through them to the slot element beneath.
  * - DragOverlay has pointer-events:none natively (dnd-kit).
  * - RackSlot divs carry data-rack-id / data-u-pos attributes that we read here.
- *
- * This is intentionally independent of dnd-kit's collision detection, which
- * cannot reliably distinguish 44px slot elements from large (~1800px) rack
- * column sortable containers that share the same DndContext.
  */
 function resolveSlotFromPoint(clientX: number, clientY: number): HoverSlot | null {
   const el = document.elementFromPoint(clientX, clientY);
@@ -99,6 +95,10 @@ export function RackPlanner() {
   const [hoverSlot, setHoverSlot] = useState<HoverSlot | null>(null);
   const hoverSlotRef = useRef<HoverSlot | null>(null);
 
+  // Tracks whether ANY module/palette drag is in progress — used to
+  // activate the body.rack-dragging CSS class and the pointermove listener.
+  const isDraggingAnyRef = useRef(false);
+
   function updateHoverSlot(slot: HoverSlot | null) {
     hoverSlotRef.current = slot;
     setHoverSlot(slot);
@@ -112,36 +112,47 @@ export function RackPlanner() {
     fetchRacks().catch(() => toast.error('Failed to load racks'));
   }, [fetchRacks]);
 
+  /**
+   * Native pointermove listener registered once on mount.
+   * Only runs while isDraggingAnyRef is true — gives us the exact cursor
+   * position without any reconstruction arithmetic, so resolveSlotFromPoint
+   * is always called with accurate coordinates.
+   */
+  useEffect(() => {
+    function onPointerMove(e: PointerEvent) {
+      if (!isDraggingAnyRef.current) return;
+      const slot = resolveSlotFromPoint(e.clientX, e.clientY);
+      hoverSlotRef.current = slot;
+      setHoverSlot(slot);
+    }
+
+    // Capture phase so we get the event before any element can stop propagation.
+    window.addEventListener('pointermove', onPointerMove, { capture: true });
+    return () => window.removeEventListener('pointermove', onPointerMove, { capture: true });
+  }, []);
+
   function handleDragStart(event: DragStartEvent) {
     const data = event.active.data.current as Record<string, unknown>;
     if (data?.dragType === 'palette') {
       setActivePaletteType(data.type as ModuleType);
+      isDraggingAnyRef.current = true;
+      document.body.classList.add('rack-dragging');
     } else if (data?.dragType === 'module') {
       setDraggingModuleId(data.moduleId as string);
       setActiveDragModuleLabel(data.label as string);
+      isDraggingAnyRef.current = true;
+      document.body.classList.add('rack-dragging');
     }
     updateHoverSlot(null);
   }
 
-  function handleDragMove(event: DragMoveEvent) {
-    const data = event.active.data.current as Record<string, unknown>;
-    // Only track slot hover for module and palette drags
-    if (data?.dragType !== 'module' && data?.dragType !== 'palette') {
-      updateHoverSlot(null);
-      return;
-    }
-
-    // Compute current pointer position from the activating event + accumulated delta
-    const activatorEvent = event.activatorEvent as PointerEvent;
-    const clientX = activatorEvent.clientX + event.delta.x;
-    const clientY = activatorEvent.clientY + event.delta.y;
-
-    updateHoverSlot(resolveSlotFromPoint(clientX, clientY));
-  }
-
   async function handleDragEnd(event: DragEndEvent) {
     const { active, over } = event;
 
+    // Stop native hover tracking and remove body class FIRST
+    isDraggingAnyRef.current = false;
+    document.body.classList.remove('rack-dragging');
+
     // Capture hoverSlot BEFORE resetting state
     const slot = hoverSlotRef.current;
 
@@ -210,7 +221,6 @@ export function RackPlanner() {
       sensors={sensors}
       collisionDetection={closestCenter}
       onDragStart={handleDragStart}
-      onDragMove={handleDragMove}
       onDragEnd={handleDragEnd}
     >
       <div className="flex flex-col h-screen bg-[#0f1117]">