frontend/src/components/DraggableDroppableTagRow.tsx

@@ -58,15 +58,14 @@ const DraggableDroppableTagRow: FC<Props> = ({ tag, nestLevel, pathKey, parentTa
 						   data:  { kind:   'tag',
 							    tagId:  tag.id,
 							    parentTagId,
-							    nestLevel,
-							    dndId } })
+							    nestLevel } })
 
   const { setNodeRef: setDropRef, isOver: over } = useDroppable ({
       id:   dndId,
       data: { kind: 'tag', tagId: tag.id } })
 
   const style: CSSProperties = { transform:   CSS.Translate.toString (transform),
-				 visibility:  dragging ? 'hidden' : 'visible' }
+				 visibility:  dragging && !(sp) ? 'hidden' : 'visible' }
 
   return (
       <div
@@ -79,12 +78,16 @@ const DraggableDroppableTagRow: FC<Props> = ({ tag, nestLevel, pathKey, parentTa
 	      return
 	    const dx = e.clientX - p.x
 	    const dy = e.clientY - p.y
+
 	    if (dx * dx + dy * dy >= 9)
 	      armEatNextClick ()
 	  }}
 	  onPointerUpCapture={() => {
 	    downPosRef.current = null
 	  }}
+	  onPointerCancelCapture={() => {
+	    downPosRef.current = null
+	  }}
 	  onClickCapture={e => {
 	    if (suppressClickRef.current)
 	      {
@@ -100,7 +103,6 @@ const DraggableDroppableTagRow: FC<Props> = ({ tag, nestLevel, pathKey, parentTa
 	  >
 	<motion.div
 	    ref={setDragRef}
-	    data-tag-dnd-id={dndId}
 	    style={style}
 	    className={cn (
 		'inline-flex min-w-0 max-w-full items-baseline overflow-hidden',

frontend/src/components/TagDetailSidebar.tsx

@@ -1,8 +1,8 @@
 import { DndContext,
 	 DragOverlay,
+	 MeasuringStrategy,
 	 MouseSensor,
 	 TouchSensor,
-	 type Modifier,
 	 pointerWithin,
 	 rectIntersection,
 	 useDroppable,
@@ -11,7 +11,7 @@ import { DndContext,
 import { restrictToWindowEdges } from '@dnd-kit/modifiers'
 import { useQueryClient } from '@tanstack/react-query'
 import { motion } from 'framer-motion'
-import { useEffect, useLayoutEffect, useMemo, useRef, useState } from 'react'
+import { useEffect, useMemo, useRef, useState } from 'react'
 
 import DraggableDroppableTagRow from '@/components/DraggableDroppableTagRow'
 import PrefetchLink from '@/components/PrefetchLink'
@@ -28,13 +28,21 @@ import { postsKeys, tagsKeys } from '@/lib/queryKeys'
 import { useClientBehaviourSettings } from '@/lib/useClientBehaviourSettings'
 import { dateString, originalCreatedAtString } from '@/lib/utils'
 
-import type { DragEndEvent } from '@dnd-kit/core'
+import type { CollisionDetection, DragEndEvent } from '@dnd-kit/core'
 import type { FC, MutableRefObject, ReactNode } from 'react'
 
 import type { Category, Post, TagWithSections } from '@/types'
 
 type TagByCategory = { [key in Category]: TagWithSections[] }
 
+const tagCollisionDetection: CollisionDetection = args => {
+  const byPointer = pointerWithin (args)
+  return byPointer.length > 0 ? byPointer : rectIntersection (args)
+}
+
+const alwaysMeasureDroppables = {
+    droppable: { strategy: MeasuringStrategy.Always } } as const
+
 
 const renderTagTree = (
     tag:              TagWithSections,
@@ -180,6 +188,24 @@ const DropSlot = ({ cat }: { cat: Category }) => {
       </li>)
 }
 
+const EmptyCategoryDropSection = (
+    { cat, children }: { cat: Category
+			 children: ReactNode }) => {
+  const { setNodeRef, isOver: over } = useDroppable ({
+      id:   `slot:${ cat }`,
+      data: { kind: 'slot', cat } })
+
+  return (
+      <div ref={setNodeRef} className="my-3">
+	{children}
+	<ul>
+	  <li className="h-1">
+	    {over && <div className="h-0.5 w-full rounded bg-sky-400"/>}
+	  </li>
+	</ul>
+      </div>)
+}
+
 
 type Props = {
     className?: string
@@ -188,12 +214,7 @@ type Props = {
 
 type ActiveTagDrag = {
     tagId:      number
-    nestLevel:  number
-    dndId:      string }
-
-type DragOverlayOffset = {
-    x:  number
-    y:  number }
+    nestLevel:  number }
 
 
 const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
@@ -228,59 +249,16 @@ const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
   const [dragging, setDragging] = useState (false)
   const [saving, setSaving] = useState (false)
   const [tags, setTags] = useState (baseTags)
-  const [dragOverlayOffset, setDragOverlayOffset] =
-      useState<DragOverlayOffset> ({ x: 0, y: 0 })
   const flatTagsByCategory = useMemo<TagByCategory> (
       () => buildFlatTagByCategory (tags),
       [tags],
   )
 
   const suppressClickRef = useRef (false)
-  const dragStartRectRef = useRef<DOMRect | null> (null)
-  const offsetFrameRef = useRef<number | null> (null)
 
   const sensors = useSensors (
       useSensor (MouseSensor, { activationConstraint: { distance: 6 } }),
       useSensor (TouchSensor, { activationConstraint: { delay: 250, tolerance: 8 } }))
-  const dragOverlayModifiers = useMemo<Modifier[]> (
-      () => (
-	  sp
-	  ? [({ transform }) => ({
-	      ...transform,
-	      x: transform.x + dragOverlayOffset.x,
-	      y: transform.y + dragOverlayOffset.y })]
-	  : []),
-      [dragOverlayOffset.x, dragOverlayOffset.y, sp],
-  )
-
-  useLayoutEffect (() => {
-    if (!(sp && dragging && activeTagDrag && dragStartRectRef.current))
-      {
-	setDragOverlayOffset ({ x: 0, y: 0 })
-	return
-      }
-
-    offsetFrameRef.current = requestAnimationFrame (() => {
-      const node = document.querySelector<HTMLElement> (
-	  `[data-tag-dnd-id="${ activeTagDrag.dndId }"]`,
-      )
-
-      if (!(node && dragStartRectRef.current))
-	return
-
-      const currentRect = node.getBoundingClientRect ()
-      const initialRect = dragStartRectRef.current
-
-      setDragOverlayOffset ({
-	  x: currentRect.left - initialRect.left,
-	  y: currentRect.top - initialRect.top })
-    })
-
-    return () => {
-      if (offsetFrameRef.current != null)
-	cancelAnimationFrame (offsetFrameRef.current)
-    }
-  }, [activeTagDrag, dragging, sp])
 
   const reloadTags = async (): Promise<void> => {
     setTags (buildTagByCategory (await apiGet<Post> (`/posts/${ post.id }`)))
@@ -383,28 +361,17 @@ const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
 	<TagSearch/>
 	<DndContext
 	    sensors={sensors}
-	    collisionDetection={args => {
-	      const byPointer = pointerWithin (args)
-	      return byPointer.length > 0 ? byPointer : rectIntersection (args)
-	    }}
+	    collisionDetection={sp ? tagCollisionDetection : undefined}
+	    measuring={sp ? alwaysMeasureDroppables : undefined}
 	    onDragStart={e => {
 	      if (e.active.data.current?.kind === 'tag')
 		{
 		  const tagId = e.active.data.current?.tagId
 		  const nestLevel = e.active.data.current?.nestLevel
-		  const dndId = e.active.data.current?.dndId
-		  dragStartRectRef.current =
-		      e.active.rect.current.initial
-		      ? new DOMRect (
-			  e.active.rect.current.initial.left,
-			  e.active.rect.current.initial.top,
-			  e.active.rect.current.initial.width,
-			  e.active.rect.current.initial.height)
-		      : null
 		  setActiveTagDrag (
-		      tagId == null || nestLevel == null || !(dndId)
+		      tagId == null || nestLevel == null
 		      ? null
-		      : { tagId, nestLevel, dndId })
+		      : { tagId, nestLevel })
 		}
 	      setDragging (true)
 	      suppressClickRef.current = true
@@ -418,16 +385,12 @@ const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
 	    onDragCancel={() => {
 	      setActiveTagDrag (null)
 	      setDragging (false)
-	      setDragOverlayOffset ({ x: 0, y: 0 })
-	      dragStartRectRef.current = null
 	      document.body.style.userSelect = ''
 	      suppressClickRef.current = false
 	    }}
 	    onDragEnd={async e => {
 	      setActiveTagDrag (null)
 	      setDragging (false)
-	      setDragOverlayOffset ({ x: 0, y: 0 })
-	      dragStartRectRef.current = null
 	      await onDragEnd (e)
 	      document.body.style.userSelect = ''
 	    }}
@@ -441,8 +404,7 @@ const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
 	    if (!(categoryTags.length > 0 || dragging))
 	      return null
 
-	    return (
-		<div className="my-3" key={cat}>
+	    const sectionTitle = (
 		<SubsectionTitle>
 		  <motion.div
 		      layoutId={animationMode === 'off'
@@ -451,8 +413,17 @@ const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
 		      transition={{ layout: layoutTransition }}>
 		    {CATEGORY_NAMES[cat]}
 		  </motion.div>
-		  </SubsectionTitle>
+		</SubsectionTitle>)
 
+	    if (!(categoryTags.length > 0))
+	      return (
+		  <EmptyCategoryDropSection cat={cat} key={cat}>
+		    {sectionTitle}
+		  </EmptyCategoryDropSection>)
+
+	    return (
+		<div className="my-3" key={cat}>
+		  {sectionTitle}
 		  <ul>
 		    {(tagRelationDisplay === 'grouped'
 		      ? (tags[cat] ?? []).flatMap (tag =>
@@ -520,16 +491,15 @@ const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
 		</ul>
 	      </motion.div>)}
 
-	  <DragOverlay adjustScale={false} modifiers={dragOverlayModifiers}>
+	  {!(sp) && (
+	      <DragOverlay adjustScale={false}>
 		<div className="pointer-events-none">
 		  {activeTagDrag != null && (() => {
 		    const tag = findTag (tags, activeTagDrag.tagId)
-		return (
-		    tag
-		    && <TagLink tag={tag} nestLevel={activeTagDrag.nestLevel}/>)
+		    return tag && <TagLink tag={tag} nestLevel={activeTagDrag.nestLevel}/>
 		  }) ()}
 		</div>
-	  </DragOverlay>
+	      </DragOverlay>)}
 	</DndContext>
       </SidebarComponent>)
 }