Keep an unsent message as a draft while switching rooms

The composer stores what's being written in localStorage per room and
restores it when the room is opened again. Sending clears it.

app/javascript/controllers/composer_controller.js

@@ -12,11 +12,21 @@ export default class extends Controller {
   #files = []
 
   connect() {
+    this.#restoreDraft()
+
     if (!this.#usingTouchDevice) {
       onNextEventLoopTick(() => this.textTarget.focus())
     }
   }
 
+  saveDraft() {
+    if (this.textTarget.isBlank) {
+      localStorage.removeItem(this.#draftKey)
+    } else {
+      localStorage.setItem(this.#draftKey, this.textTarget.value)
+    }
+  }
+
   submit(event) {
     event.preventDefault()
 
@@ -107,6 +117,19 @@ export default class extends Controller {
     this.fieldsTarget.disabled = true
   }
 
+  #restoreDraft() {
+    const draft = localStorage.getItem(this.#draftKey)
+
+    if (draft) {
+      this.textTarget.value = draft
+      this.textTarget.selection.placeCursorAtTheEnd()
+    }
+  }
+
+  get #draftKey() {
+    return `composer-draft-${this.roomIdValue}`
+  }
+
   get #usingTouchDevice() {
     return 'ontouchstart' in window || navigator.maxTouchPoints > 0 || navigator.msMaxTouchPoints > 0;
   }
@@ -158,6 +181,7 @@ export default class extends Controller {
 
   #reset() {
     this.textTarget.value = ""
+    localStorage.removeItem(this.#draftKey)
   }
 
   #updateFileList() {

app/views/rooms/show/_composer.html.erb

@@ -25,7 +25,7 @@
                         "permitted-attachment-types": "application/vnd.campfire.mention application/vnd.actiontext.opengraph-embed",
                         data: {
                           controller: "unfurl",
-                          action: "#{rich_text_data_actions} lexxy:insert-link->unfurl#unfurl",
+                          action: "#{rich_text_data_actions} lexxy:change->composer#saveDraft lexxy:insert-link->unfurl#unfurl",
                           composer_target: "text" } do %>
                     <%= mention_prompt_tag room %>
                   <% end %>

test/system/composer_test.rb

@@ -29,6 +29,27 @@ class ComposerTest < ApplicationSystemTestCase
     assert_composer_empty
   end
 
+  test "an unsent message is kept as a draft while hopping between rooms" do
+    type_in_composer "Still writing this"
+
+    join_room rooms(:hq)
+    assert_composer_empty
+
+    type_in_composer "And this one too"
+
+    join_room rooms(:designers)
+    assert_composer_text "Still writing this"
+
+    press_in_composer :enter
+    assert_message_text "Still writing this"
+
+    join_room rooms(:hq)
+    assert_composer_text "And this one too"
+
+    join_room rooms(:designers)
+    assert_composer_empty
+  end
+
   test "markdown strikethrough survives sanitization" do
     type_in_composer "Hello ~~Claude~~ World"
     press_in_composer :enter