fix: improve DND drag feedback in EditableSchemaForm (#8449)

Three issues fixed:
- Dragged element clone was invisible because morphDraggedElementToBeLike
  ran before the clone was in the DOM, copying 0-height from the
  uninitialized ResizeTransitionWrapper shadow. Fixed with morphDisabled.
- Shadow placeholder was inconsistently hidden because the DND library's
  inline visibility:hidden was overwritten by RTW's reactive style binding.
  Fixed with !visible CSS class that overrides inline styles.
- Small cursor movements immediately triggered field reordering. Added a
  200ms grace period after drag start before processing reorder events.

The shadow element now shows a dashed blue drop-target indicator instead
of being fully hidden.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/SchemaForm.svelte

@@ -16,7 +16,11 @@
 	import ArgInput from './ArgInput.svelte'
 	import { createEventDispatcher, untrack } from 'svelte'
 	import { deepEqual } from 'fast-equals'
-	import { dragHandleZone, type Options as DndOptions } from '@windmill-labs/svelte-dnd-action'
+	import {
+		dragHandleZone,
+		SHADOW_ITEM_MARKER_PROPERTY_NAME,
+		type Options as DndOptions
+	} from '@windmill-labs/svelte-dnd-action'
 	import type { SchemaDiff } from '$lib/components/schema/schemaUtils.svelte'
 	import type { ComponentCustomCSS } from './apps/types'
 	import ResizeTransitionWrapper from './common/ResizeTransitionWrapper.svelte'
@@ -295,7 +299,9 @@
 				class={twMerge(
 					typeof diff[argName] === 'object' &&
 						diff[argName].diff !== 'same' &&
-						'bg-red-300 dark:bg-red-800 rounded-md'
+						'bg-red-300 dark:bg-red-800 rounded-md',
+					item[SHADOW_ITEM_MARKER_PROPERTY_NAME] &&
+						'!visible border-2 border-dashed border-blue-300 dark:border-blue-600 bg-blue-50 dark:bg-blue-900/20 rounded-md [&>*]:invisible'
 				)}
 				innerClass="w-full"
 			>

frontend/src/lib/components/schema/SchemaFormDND.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { createEventDispatcher, untrack } from 'svelte'
-	import { dragHandle } from '@windmill-labs/svelte-dnd-action'
+	import { dragHandle, TRIGGERS } from '@windmill-labs/svelte-dnd-action'
 	import SchemaForm from '../SchemaForm.svelte'
 	import { GripVertical } from 'lucide-svelte'
 	import type { Schema } from '$lib/common'
@@ -79,9 +79,24 @@
 		}
 	}
 
+	let dragStartTime = 0
+	const DRAG_GRACE_PERIOD_MS = 200
+
 	function handleConsider(e) {
 		dragDisabledState = false
-		const { items: newItems } = e.detail
+		const { items: newItems, info } = e.detail
+
+		if (info.trigger === TRIGGERS.DRAG_STARTED) {
+			dragStartTime = Date.now()
+			items = $state.snapshot(newItems)
+			return
+		}
+
+		// Ignore reorders during grace period so small movements don't cause jumps
+		if (Date.now() - dragStartTime < DRAG_GRACE_PERIOD_MS) {
+			return
+		}
+
 		items = $state.snapshot(newItems)
 	}
 
@@ -130,7 +145,8 @@
 				items,
 				flipDurationMs,
 				dropTargetStyle: {},
-				type: dndType
+				type: dndType,
+				morphDisabled: true
 			}}
 	{items}
 	{diff}