flutter_browser_tabs_example: a tab pressed in the title bar band drags the tab

The tab strip sits under the macOS title bar (content under the title bar),
and AppKit moves the window for any press in that 28 pt band before Flutter
sees it, so grabbing a tab by its upper half dragged the whole window
instead of the tab. The strip moves the window itself through its drag
session (SetPosition), so the window is made not movable by the system,
as core already does for TitleBarStyle.hidden.

Checked on macOS: a tab grabbed in the band reorders and the window stays;
the empty strip still moves the window; a tab pulled down still tears off.

Author: lijy91

examples/flutter_browser_tabs_example/lib/src/tabs_controller.dart

@@ -131,7 +131,13 @@ class TabsController extends ChangeNotifier {
     // is a title bar the content has taken in; elsewhere the strip carries its own
     // close button and the title bar goes away with its buttons.
     if (native != null) {
-      if (!native.setContentUnderTitleBar(true)) {
+      if (native.setContentUnderTitleBar(true)) {
+        // macOS still moves the window for any press in the title bar band
+        // (the top 28 pt), tabs included, before Flutter sees it. The strip
+        // moves the window itself, so take that away from the system; with
+        // TitleBarStyle.hidden core already does.
+        native.isMovable = false;
+      } else {
         native.titleBarStyle = na.TitleBarStyle.hidden;
       }
       // Either way the frame is kept, so the content just grew into the title