Refresh input when placeholder reactive is updated

src/textual/widgets/_input.py

@@ -176,7 +176,7 @@ class Input(Widget, can_focus=True):
     input_scroll_offset = reactive(0)
     cursor_position = reactive(0)
     view_position = reactive(0)
-    placeholder = reactive("")
+    placeholder = reactive("", init=False)
     complete = reactive("")
     width = reactive(1)
     _cursor_visible = reactive(True)
@@ -394,6 +394,9 @@ def _watch_cursor_blink(self, blink: bool) -> None:
                 self._cursor_visible = True
                 self._blink_timer.pause()
 
+    def _watch_placeholder(self) -> None:
+        self.refresh()
+
     @property
     def cursor_screen_offset(self) -> Offset:
         """The offset of the cursor of this input in screen-space. (x, y)/(column, row)"""