fix #6523 (#6524)

app/javascript/mastodon/reducers/compose.js

@@ -34,7 +34,7 @@ import uuid from '../uuid';
 import { me } from '../initial_state';
 
 const initialState = ImmutableMap({
-  mounted: false,
+  mounted: 0,
   sensitive: false,
   spoiler: false,
   spoiler_text: '',
@@ -159,10 +159,10 @@ export default function compose(state = initialState, action) {
   case STORE_HYDRATE:
     return hydrate(state, action.state.get('compose'));
   case COMPOSE_MOUNT:
-    return state.set('mounted', true);
+    return state.set('mounted', state.get('mounted') + 1);
   case COMPOSE_UNMOUNT:
     return state
-      .set('mounted', false)
+      .set('mounted', Math.max(state.get('mounted') - 1, 0))
       .set('is_composing', false);
   case COMPOSE_SENSITIVITY_CHANGE:
     return state.withMutations(map => {