this solution looks promising

Riot/Modules/Room/Views/InputToolbar/RoomInputToolbarView.h

@@ -75,6 +75,8 @@ typedef NS_ENUM(NSUInteger, RoomInputToolbarViewSendMode)
  */
 - (void)roomInputToolbarView:(RoomInputToolbarView *)toolbarView sendAttributedTextMessage:(NSAttributedString *)attributedTextMessage;
 
+- (void)didChangeMaximisedState: (BOOL) state;
+
 @end
 
 /**

Riot/Modules/Room/Views/WYSIWYGInputToolbar/WysiwygInputToolbarView.swift

@@ -111,6 +111,12 @@ class WysiwygInputToolbarView: MXKRoomInputToolbarView, NibLoadable, HtmlRoomInp
                 .removeDuplicates()
                 .sink { [weak hostingViewController] _ in
                     hostingViewController?.view.setNeedsLayout()
+                },
+            
+            wysiwygViewModel.$maximised
+                .removeDuplicates()
+                .sink { [weak self] value in
+                    self?.toolbarViewDelegate?.didChangeMaximisedState(value)
                 }
         ]