diff --git a/frontend/libs/components/promptbox/src/lib/MentionTextarea.tsx b/frontend/libs/components/promptbox/src/lib/MentionTextarea.tsx
index d35fbf1a01..3f00aad490 100644
--- a/frontend/libs/components/promptbox/src/lib/MentionTextarea.tsx
+++ b/frontend/libs/components/promptbox/src/lib/MentionTextarea.tsx
@@ -1,4 +1,4 @@
-import { useCallback, useEffect, useMemo, useRef, useState } from "react";
+import { forwardRef, useCallback, useEffect, useImperativeHandle, useMemo, useRef, useState } from "react";
 import { twMerge } from "tailwind-merge";
 import { FontAwesomeIcon } from "@fortawesome/react-fontawesome";
 import { faUser } from "@fortawesome/pro-solid-svg-icons";
@@ -24,7 +24,6 @@ interface MentionTextareaProps {
   onBlur?: () => void;
   disabled?: boolean;
   colorMap: Record<string, string>;
-  maxHeight?: number;
 }
 
 interface MentionState {
@@ -241,7 +240,7 @@ function MentionDropdown({
 // MentionTextarea Component
 // ---------------------------------------------------------------------------
 
-export function MentionTextarea({
+export const MentionTextarea = forwardRef<HTMLDivElement, MentionTextareaProps>(function MentionTextarea({
   value,
   onChange,
   mentionItems,
@@ -252,9 +251,9 @@ export function MentionTextarea({
   onBlur,
   disabled,
   colorMap,
-  maxHeight,
-}: MentionTextareaProps) {
+}, ref) {
   const editorRef = useRef<HTMLDivElement>(null);
+  useImperativeHandle(ref, () => editorRef.current!, []);
   const isInternalUpdate = useRef(false);
   const isComposing = useRef(false);
   const pendingCaret = useRef<number | null>(null);
@@ -498,6 +497,17 @@ export function MentionTextarea({
           return;
         }
       }
+
+      // Shift+Enter (or Cmd+Enter on Mac): insert a newline instead of
+      // letting the contentEditable create a <div>
+      if (e.key === "Enter" && (e.shiftKey || e.metaKey)) {
+        e.preventDefault();
+        document.execCommand("insertLineBreak");
+        // Trigger input handler so the parent value stays in sync
+        handleInput();
+        return;
+      }
+
       externalOnKeyDown?.(e);
     },
     [
@@ -506,6 +516,7 @@ export function MentionTextarea({
       filteredItems,
       handleSelect,
       externalOnKeyDown,
+      handleInput,
     ],
   );
 
@@ -553,7 +564,6 @@ export function MentionTextarea({
           className,
           "outline-none whitespace-pre-wrap break-words overflow-y-auto",
         )}
-        style={maxHeight ? { maxHeight } : undefined}
       />
 
       {mentionState.isOpen && filteredItems.length > 0 && (
@@ -569,4 +579,4 @@ export function MentionTextarea({
       )}
     </div>
   );
-}
+});
diff --git a/frontend/libs/components/promptbox/src/lib/PromptBoxVideo.tsx b/frontend/libs/components/promptbox/src/lib/PromptBoxVideo.tsx
index ccb029a272..0ac2096f12 100644
--- a/frontend/libs/components/promptbox/src/lib/PromptBoxVideo.tsx
+++ b/frontend/libs/components/promptbox/src/lib/PromptBoxVideo.tsx
@@ -172,8 +172,9 @@ export const PromptBoxVideo = ({
   const toggleExpand = () => {
     setIsExpanded((prev) => {
       const next = !prev;
-      if (textareaRef.current) {
-        textareaRef.current.style.height = next ? EXPANDED_HEIGHT : "auto";
+      const el = (mentionEditorRef.current ?? textareaRef.current) as HTMLElement | null;
+      if (el) {
+        el.style.height = next ? EXPANDED_HEIGHT : "auto";
       }
       return next;
     });
@@ -226,17 +227,18 @@ export const PromptBoxVideo = ({
     useState<PopoverItem[]>(aspectRatioOptions);
 
   const textareaRef = useRef<HTMLTextAreaElement>(null);
+  const mentionEditorRef = useRef<HTMLDivElement>(null);
 
+  // Apply height constraints to whichever editor element is active
   useEffect(() => {
-    if (textareaRef.current) {
-      // Hard pixel limit so the resize handle can never exceed viewport
+    const el = (mentionEditorRef.current ?? textareaRef.current) as HTMLElement | null;
+    if (el) {
       const maxH = isExpanded ? 500 : Math.min(window.innerHeight - 700, 500);
-      textareaRef.current.style.maxHeight = `${Math.max(maxH, 88)}px`;
-      textareaRef.current.style.minHeight = "0";
+      el.style.maxHeight = `${Math.max(maxH, 88)}px`;
+      el.style.minHeight = "0";
       if (!isExpanded) {
-        // Cap auto-grow at ~5.5em so it doesn't fight with manual resize
-        const capped = Math.min(textareaRef.current.scrollHeight, 88);
-        textareaRef.current.style.minHeight = `${capped}px`;
+        const capped = Math.min(el.scrollHeight, 88);
+        el.style.minHeight = `${capped}px`;
       }
     }
   });
@@ -894,11 +896,11 @@ export const PromptBoxVideo = ({
             <div className="promptbox-resize-wrap relative flex-1">
               {hasAnyMentionables ? (
                 <MentionTextarea
+                  ref={mentionEditorRef}
                   value={prompt}
                   onChange={setPrompt}
                   mentionItems={allMentionItems}
                   colorMap={mentionColorMap}
-                  maxHeight={isExpanded ? 500 : Math.max(Math.min(window.innerHeight - 700, 500), 88)}
                   placeholder={
                     isReferenceMode
                       ? "Use @Image1, @Video1, @Audio1... to reference uploads in prompt..."