Survive monitors being replaced, and report a primary one on Wayland

Display kept a bare GdkMonitor pointer. A GdkMonitor belongs to its GdkDisplay
and is dropped and recreated on hotplug and whenever the compositor powers the
outputs off and on, which left cached Display objects pointing at freed memory:
GDK_IS_MONITOR assertions then fired on every getter (hundreds in a single run
after the screen had blanked).

Clear the pointer through a weak pointer so the getters fall back to their
defaults, and make IsPrimary() use the first monitor when the display has no
primary one, as it never does on Wayland.

Author: lijy91

src/platform/linux/display_linux.cpp

@@ -9,16 +9,32 @@ namespace nativeapi {
 class Display::Impl {
  public:
   Impl() = default;
-  Impl(GdkMonitor* monitor) : gdk_monitor_(monitor) {}
+  Impl(GdkMonitor* monitor) { SetMonitor(monitor); }
+
+  ~Impl() {
+    if (gdk_monitor_) {
+      g_object_remove_weak_pointer(G_OBJECT(gdk_monitor_), (gpointer*)&gdk_monitor_);
+    }
+  }
+
+  // A GdkMonitor is owned by its GdkDisplay, which drops and recreates monitors
+  // on hotplug and whenever the compositor turns the outputs off and on again.
+  // Let GDK clear the pointer so the getters below report defaults instead of
+  // reading freed memory.
+  void SetMonitor(GdkMonitor* monitor) {
+    if (!monitor) {
+      return;
+    }
+    gdk_monitor_ = monitor;
+    g_object_add_weak_pointer(G_OBJECT(gdk_monitor_), (gpointer*)&gdk_monitor_);
+  }
 
   const DisplayId id_ = IdAllocator::Allocate<Display>();
   GdkMonitor* gdk_monitor_ = nullptr;
 };
 
 Display::Display(void* display) : pimpl_(std::make_unique<Impl>()) {
-  if (display) {
-    pimpl_->gdk_monitor_ = (GdkMonitor*)display;
-  }
+  pimpl_->SetMonitor((GdkMonitor*)display);
 }
 
 Display::~Display() = default;
@@ -75,6 +91,11 @@ bool Display::IsPrimary() const {
     return false;
   GdkDisplay* display = gdk_monitor_get_display(pimpl_->gdk_monitor_);
   GdkMonitor* primary = gdk_display_get_primary_monitor(display);
+  if (!primary) {
+    // Wayland has no notion of a primary monitor; match the first-monitor
+    // convention DisplayManager::EnumerateNativeDisplays() uses.
+    primary = gdk_display_get_monitor(display, 0);
+  }
   return primary == pimpl_->gdk_monitor_;
 }