diff --git a/src/components/tolariaEditorFormatting.behavior.test.tsx b/src/components/tolariaEditorFormatting.behavior.test.tsx
index cc8853be..4b007c9d 100644
--- a/src/components/tolariaEditorFormatting.behavior.test.tsx
+++ b/src/components/tolariaEditorFormatting.behavior.test.tsx
@@ -287,6 +287,29 @@ describe('tolariaEditorFormatting behavior', () => {
     }))
   })
 
+  it('adds viewport-clamping middleware and preserves caller middleware', () => {
+    const editor = createMockEditor('paragraph')
+    useBlockNoteEditorMock.mockReturnValue(editor)
+
+    render(
+      <TolariaFormattingToolbarController
+        floatingUIOptions={{
+          useFloatingOptions: {
+            middleware: [{ name: 'custom-middleware' } as never],
+          },
+        }}
+      />,
+    )
+
+    const floatingOptions = positionPopoverState.lastProps?.useFloatingOptions as {
+      middleware: Array<{ name: string }>
+    }
+
+    expect(floatingOptions.middleware.map((middleware) => middleware.name)).toEqual(
+      expect.arrayContaining(['custom-middleware', 'tolariaViewportClamp']),
+    )
+  })
+
   it('falls back to top-start and focuses the block type trigger on mouse down', () => {
     const editor = createMockEditor('paragraph')
     const focusSpy = vi.spyOn(HTMLButtonElement.prototype, 'focus').mockImplementation(() => {})
diff --git a/src/components/tolariaEditorFormatting.tsx b/src/components/tolariaEditorFormatting.tsx
index 4307711c..8c0ace45 100644
--- a/src/components/tolariaEditorFormatting.tsx
+++ b/src/components/tolariaEditorFormatting.tsx
@@ -67,6 +67,9 @@ import {
 type TolariaBasicTextStyle = 'bold' | 'italic' | 'strike' | 'code'
 
 const FORMATTER_CLOSE_GRACE_MS = 160
+const FORMATTER_VIEWPORT_PADDING_PX = 8
+type TolariaFloatingOptions = NonNullable<FloatingUIOptions['useFloatingOptions']>
+type TolariaFloatingMiddleware = NonNullable<TolariaFloatingOptions['middleware']>[number]
 
 function isFocusStillWithinToolbar(
   currentTarget: EventTarget & Element,
@@ -230,6 +233,42 @@ function textAlignmentToPlacement(
   }
 }
 
+function viewportClampMiddleware(): TolariaFloatingMiddleware {
+  return {
+    name: 'tolariaViewportClamp',
+    fn({ x, rects }: { rects: { floating: { width: number } }; x: number }) {
+      const viewportWidth = window.visualViewport?.width ?? window.innerWidth
+      const minX = FORMATTER_VIEWPORT_PADDING_PX
+      const maxX = Math.max(
+        minX,
+        viewportWidth - rects.floating.width - FORMATTER_VIEWPORT_PADDING_PX,
+      )
+
+      return {
+        x: Math.min(Math.max(x, minX), maxX),
+      }
+    },
+  }
+}
+
+function withViewportSafeMiddleware(
+  options?: TolariaFloatingOptions,
+): TolariaFloatingOptions {
+  if (!options) {
+    return {
+      middleware: [viewportClampMiddleware()],
+    }
+  }
+
+  return {
+    ...options,
+    middleware: [
+      ...(options.middleware ?? []),
+      viewportClampMiddleware(),
+    ],
+  }
+}
+
 function editorSupportsTextStyle(
   style: TolariaBasicTextStyle,
   editor: BlockNoteEditor<BlockSchema, InlineContentSchema, StyleSchema>,
@@ -730,7 +769,7 @@ export function TolariaFormattingToolbarController(props: {
           }
         },
         placement,
-        ...props.floatingUIOptions?.useFloatingOptions,
+        ...withViewportSafeMiddleware(props.floatingUIOptions?.useFloatingOptions),
       },
       elementProps: {
         style: {