Add full support for hardware keyboards (Chromebook, Android tablets, etc) that don't alter screen UI (#765)

lib/src/widgets/raw_editor.dart

@@ -577,6 +577,8 @@ class RawEditorState extends EditorState
               _onChangeTextEditingValue(!_hasFocus);
             }
           });
+
+          HardwareKeyboard.instance.addHandler(_hardwareKeyboardEvent);
         }
       });
     }
@@ -585,6 +587,27 @@ class RawEditorState extends EditorState
     widget.focusNode.addListener(_handleFocusChanged);
   }
 
+  // KeyboardVisibilityController only checks for keyboards that
+  // adjust the screen size. Also watch for hardware keyboards
+  // that don't alter the screen (i.e. Chromebook, Android tablet
+  // and any hardware keyboards from an OS not listed in isKeyboardOS())
+  bool _hardwareKeyboardEvent(KeyEvent _) {
+    if (!_keyboardVisible) {
+      // hardware keyboard key pressed. Set visibility to true
+      _keyboardVisible = true;
+      // update the editor
+      _onChangeTextEditingValue(!_hasFocus);
+    }
+
+    // remove the key handler - it's no longer needed. If
+    // KeyboardVisibilityController clears visibility, it wil
+    // also enable it when appropriate.
+    HardwareKeyboard.instance.removeHandler(_hardwareKeyboardEvent);
+
+    // we didn't handle the event, just needed to know a key was pressed
+    return false;
+  }
+
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
@@ -657,6 +680,7 @@ class RawEditorState extends EditorState
   void dispose() {
     closeConnectionIfNeeded();
     _keyboardVisibilitySubscription?.cancel();
+    HardwareKeyboard.instance.removeHandler(_hardwareKeyboardEvent);
     assert(!hasConnection);
     _selectionOverlay?.dispose();
     _selectionOverlay = null;