Add Thunderbird-style selective quote on reply

app/controllers/drafts_controller.rb

@@ -42,7 +42,7 @@ def create
         identity: identity,
         sender_alias: sender,
         subject: build_default_subject(parent),
-        body: ""
+        body: build_quoted_body(parent, params[:selected_text])
       )
     rescue ActiveRecord::RecordNotUnique
       current_user.outgoing_drafts
@@ -139,6 +139,16 @@ def build_default_subject(parent)
     "Re: #{base}"
   end
 
+  def build_quoted_body(parent, selected_text)
+    return "" if selected_text.blank?
+
+    display = parent.sender_display_alias
+    date_str = parent.created_at.strftime("%a, %d %b %Y")
+    header = "On #{date_str}, #{display.name} <#{display.email}> wrote:"
+    quoted = selected_text.strip.each_line.map { |l| "> #{l.chomp}" }.join("\n")
+    "#{header}\n#{quoted}\n\n"
+  end
+
   def active_settings_section
     :my_emails
   end

app/javascript/controllers/reply_quote_controller.js

@@ -0,0 +1,35 @@
+import { Controller } from "@hotwired/stimulus"
+
+export default class extends Controller {
+  connect() {
+    this._stored = ""
+    this._onMouseUp = () => { this._stored = this._selectionInBody() }
+    this.element.addEventListener("mouseup", this._onMouseUp)
+  }
+
+  disconnect() {
+    this.element.removeEventListener("mouseup", this._onMouseUp)
+  }
+
+  injectSelection(event) {
+    const text = this._stored || this._selectionInBody()
+    this._stored = ""
+    if (!text) return
+
+    const input = document.createElement("input")
+    input.type = "hidden"
+    input.name = "selected_text"
+    input.value = text
+    event.target.appendChild(input)
+  }
+
+  _selectionInBody() {
+    const sel = window.getSelection()
+    if (!sel || sel.isCollapsed) return ""
+    const range = sel.getRangeAt(0)
+    const body = this.element.querySelector(".message-body")
+    return (body && body.contains(range.commonAncestorContainer))
+      ? sel.toString().trim()
+      : ""
+  }
+}

app/views/topics/_message.html.slim

@@ -3,7 +3,7 @@
 - collapse_read = local_assigns.fetch(:collapse_read, true)
 - is_collapsed = (render_mode == :collapsed) || (render_mode == :shell && is_read && collapse_read)
 - message_classes = [("reply-message" if message.reply_to_id), ("message-branch-#{local_assigns[:branch_index]}" unless local_assigns[:branch_index].nil?), ("is-collapsed" if is_collapsed)].compact.join(" ")
-.message-card id=message_dom_id(message) class=message_classes data-controller="message-collapse" data-message-collapse-collapsed-value=is_collapsed data-message-id=message.id data-read=(is_read ? "true" : "false")
+.message-card id=message_dom_id(message) class=message_classes data-controller="message-collapse reply-quote" data-message-collapse-collapsed-value=is_collapsed data-message-id=message.id data-read=(is_read ? "true" : "false")
   - if (mid_anchor = message_id_anchor(message))
     a.message-id-anchor id=mid_anchor aria-hidden="true"
   - if local_assigns[:is_first_unread]
@@ -68,7 +68,7 @@
         - elsif can_reply
           = button_to drafts_path, params: { reply_to_message_id: message.id },
                       method: :post,
-                      form: { data: { turbo_frame: "draft-#{message.id}" } },
+                      form: { data: { turbo_frame: "draft-#{message.id}", action: "submit->reply-quote#injectSelection" } },
                       class: "message-action message-action-form",
                       title: "Reply", "aria-label": "Reply" do
             i.fa-solid.fa-reply