feat(ui): add smart dropdown placement to block editor menus

- introduce `useDropdownPlacement` hook to compute above/below positioning based on available viewport space
- apply dynamic `maxHeight` and position class to `BlockInsertMenu` and `BlockActionsMenu` panels
- attach `triggerRef` to menu trigger buttons for accurate anchor rect calculation
- use `useLayoutEffect` to avoid layout flicker on open

src/shared/components/BlockEditor/Block.client.js

@@ -1,10 +1,39 @@
 'use client';
 
-import React, { useEffect, useImperativeHandle, useMemo, useRef, useState, forwardRef } from 'react';
+import React, { useEffect, useImperativeHandle, useLayoutEffect, useMemo, useRef, useState, forwardRef } from 'react';
 import { Add01Icon, ArrowRight01Icon, Copy01Icon, Delete02Icon, DragDropVerticalIcon } from '@zen/core/shared/icons';
 
 // Style « boîte » pour l'icône d'un type de bloc, repris du SlashMenu.
 const TYPE_ICON_BOX_CLASS = 'w-8 h-8 flex items-center justify-center rounded-md border border-neutral-200 dark:border-neutral-700 text-xs font-medium text-neutral-700 dark:text-neutral-300 flex-shrink-0';
+
+const DROPDOWN_MAX_HEIGHT = 360;
+const DROPDOWN_MIN_HEIGHT = 160;
+const DROPDOWN_VIEWPORT_MARGIN = 8;
+const DROPDOWN_GAP = 6;
+
+// Décide si un dropdown ancré au `triggerRef` doit s'ouvrir au-dessus ou en
+// dessous, et calcule le maxHeight disponible. Recalculé chaque fois que le
+// menu s'ouvre (anchor stable tant qu'il est ouvert). Quand il s'ouvre vers
+// le haut, le contenu est ancré par `bottom: 100%` côté caller — comme ça
+// le bas reste collé au bouton et le contenu rétrécit vers le haut.
+function useDropdownPlacement(open, triggerRef) {
+  const [placement, setPlacement] = useState({ side: 'below', maxHeight: DROPDOWN_MAX_HEIGHT });
+  useLayoutEffect(() => {
+    if (!open) return;
+    const el = triggerRef.current;
+    if (!el || typeof window === 'undefined') return;
+    const rect = el.getBoundingClientRect();
+    const vh = window.innerHeight;
+    const spaceBelow = vh - rect.bottom - DROPDOWN_VIEWPORT_MARGIN - DROPDOWN_GAP;
+    const spaceAbove = rect.top - DROPDOWN_VIEWPORT_MARGIN - DROPDOWN_GAP;
+    if (spaceBelow >= Math.min(DROPDOWN_MAX_HEIGHT, DROPDOWN_MIN_HEIGHT) || spaceBelow >= spaceAbove) {
+      setPlacement({ side: 'below', maxHeight: Math.max(DROPDOWN_MIN_HEIGHT, Math.min(DROPDOWN_MAX_HEIGHT, spaceBelow)) });
+    } else {
+      setPlacement({ side: 'above', maxHeight: Math.max(DROPDOWN_MIN_HEIGHT, Math.min(DROPDOWN_MAX_HEIGHT, spaceAbove)) });
+    }
+  }, [open, triggerRef]);
+  return placement;
+}
 import { getBlockDef, listBlocks } from './blockRegistry.js';
 import { inlineLength } from './inline/types.js';
 import { inlineToDom, domToInline } from './inline/serialize.js';
@@ -31,6 +60,8 @@ function BlockInsertMenu({
   onInsert,
 }) {
   const containerRef = useRef(null);
+  const triggerRef = useRef(null);
+  const { side, maxHeight } = useDropdownPlacement(open, triggerRef);
 
   useEffect(() => {
     if (!open) return;
@@ -62,9 +93,12 @@ function BlockInsertMenu({
     };
   }
 
+  const panelPositionClass = side === 'above' ? 'bottom-full mb-1' : 'top-full mt-1';
+
   return (
     <div ref={containerRef} className="relative">
       <button
+        ref={triggerRef}
         type="button"
         tabIndex={-1}
         title="Insérer un bloc"
@@ -76,8 +110,11 @@ function BlockInsertMenu({
       </button>
 
       {open && (
-        <div className="absolute left-0 mt-1 w-64 rounded-xl border border-black/8 dark:border-white/8 bg-white dark:bg-[#0B0B0B] shadow-lg z-50">
-          <div className="p-1.5 flex flex-col gap-0.5 max-h-80 overflow-y-auto">
+        <div
+          className={`absolute left-0 ${panelPositionClass} w-64 rounded-xl border border-black/8 dark:border-white/8 bg-white dark:bg-[#0B0B0B] shadow-lg z-50 flex flex-col`}
+          style={{ maxHeight }}
+        >
+          <div className="p-1.5 flex flex-col gap-0.5 overflow-y-auto">
             <div className="px-2 pt-1 pb-1 text-[10px] font-medium uppercase tracking-wider text-neutral-400 dark:text-neutral-500">
               Insérer un bloc
             </div>
@@ -118,6 +155,8 @@ function BlockActionsMenu({
   onDelete,
 }) {
   const containerRef = useRef(null);
+  const triggerRef = useRef(null);
+  const { side, maxHeight } = useDropdownPlacement(open, triggerRef);
   const [submenuOpen, setSubmenuOpen] = useState(false);
   const submenuTimerRef = useRef(null);
 
@@ -173,9 +212,12 @@ function BlockActionsMenu({
     };
   }
 
+  const panelPositionClass = side === 'above' ? 'bottom-full mb-1' : 'top-full mt-1';
+
   return (
     <div ref={containerRef} className="relative">
       <button
+        ref={triggerRef}
         type="button"
         tabIndex={-1}
         title="Glisser pour réordonner ou cliquer pour les actions"
@@ -188,8 +230,11 @@ function BlockActionsMenu({
       </button>
 
       {open && (
-        <div className="absolute left-0 mt-1 w-56 rounded-xl border border-black/8 dark:border-white/8 bg-white dark:bg-[#0B0B0B] shadow-lg z-50">
-          <div className="p-1.5 flex flex-col gap-0.5">
+        <div
+          className={`absolute left-0 ${panelPositionClass} w-56 rounded-xl border border-black/8 dark:border-white/8 bg-white dark:bg-[#0B0B0B] shadow-lg z-50 flex flex-col`}
+          style={{ maxHeight }}
+        >
+          <div className="p-1.5 flex flex-col gap-0.5 overflow-y-auto">
             {transformOptions.length > 0 && (
               <div
                 className="relative"