Added UI force-sync button and fixed bug that didn't sync in one case… (#13745)

* Added UI force-sync button and fixed bug that didn't sync in one case and caused orange highlight

* Fix sync preset race: join old thread before starting new one

---------

Co-authored-by: Mykola Nahirnyi <mnahirnyi@amcbridge.com>
Co-authored-by: SoftFever <softfeverever@gmail.com>

src/slic3r/GUI/GUI_App.cpp

@@ -6577,8 +6577,8 @@ void GUI_App::start_sync_user_preset(bool with_progress_dlg)
                 dlg->Update(percent, _L("Loading user preset"));
             });
         };
-        cancelFn = [this, dlg]() {
-            return is_closing() || dlg->WasCanceled();
+        cancelFn = [this, dlg, t = std::weak_ptr<int>(m_user_sync_token)]() {
+            return is_closing() || dlg->WasCanceled() || t.expired();
         };
         finishFn = [this, dlg](bool) {
             CallAfter([=]{ dlg->Destroy(); });
@@ -6586,8 +6586,8 @@ void GUI_App::start_sync_user_preset(bool with_progress_dlg)
     }
     else {
         finishFn = [](bool) {}; // reload_settings() is now triggered from the background thread
-        cancelFn = [this]() {
-            return is_closing();
+        cancelFn = [this, t = std::weak_ptr<int>(m_user_sync_token)]() {
+            return is_closing() || t.expired();
         };
     }
 
@@ -6819,6 +6819,37 @@ void GUI_App::stop_sync_user_preset()
     }
 }
 
+void GUI_App::restart_sync_user_preset()
+{
+    if (!m_user_sync_token) {
+        // No sync running. If a restart helper is already in flight it will
+        // start the new sync once the old thread is joined — don't race it.
+        if (!m_restart_sync_pending)
+            start_sync_user_preset(true);
+        return;
+    }
+
+    // Resetting the token signals the old thread to stop (cancelFn checks
+    // t.expired(), so it exits after its current HTTP request completes).
+    // A helper thread joins the old thread off the UI thread — no freeze —
+    // then starts the new sync via CallAfter once the old one is fully done.
+    m_user_sync_token.reset();
+    m_restart_sync_pending = true;
+
+    auto old_thread = std::move(m_sync_update_thread);
+
+    std::thread([this, old_thread = std::move(old_thread)]() mutable {
+        if (old_thread.joinable())
+            old_thread.join();
+        m_restart_sync_pending = false;
+        if (!is_closing())
+            CallAfter([this]() {
+                if (!is_closing())
+                    start_sync_user_preset(true);
+            });
+    }).detach();
+}
+
 void GUI_App::on_stealth_mode_enter()
 {
     stop_sync_user_preset();