Fix drag-n-drop on MS-Windows

* src/w32fns.c (struct w32_drop_target): New member 'ref_count'.
(w32_drop_target_AddRef): Increment reference count.
(w32_drop_target_Release): Decrement reference count, and free the
target only if the reference count is zero.
(w32_createwindow): Initialize reference count.  (Bug#74312)

1 files changed, 6 insertions, 1 deletions

diff --git a/src/w32fns.c b/src/w32fns.c
index 1bd3d5099e2..e2455b9271e 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -2562,6 +2562,7 @@ struct w32_drop_target {
   /* i_drop_target must be the first member.  */
   IDropTarget i_drop_target;
   HWND hwnd;
+  int ref_count;
 };
 
 static HRESULT STDMETHODCALLTYPE
@@ -2573,13 +2574,16 @@ w32_drop_target_QueryInterface (IDropTarget *t, REFIID ri, void **r)
 static ULONG STDMETHODCALLTYPE
 w32_drop_target_AddRef (IDropTarget *This)
 {
-  return 1;
+  struct w32_drop_target *target = (struct w32_drop_target *) This;
+  return ++target->ref_count;
 }
 
 static ULONG STDMETHODCALLTYPE
 w32_drop_target_Release (IDropTarget *This)
 {
   struct w32_drop_target *target = (struct w32_drop_target *) This;
+  if (--target->ref_count > 0)
+    return target->ref_count;
   free (target->i_drop_target.lpVtbl);
   free (target);
   return 0;
@@ -2770,6 +2774,7 @@ w32_createwindow (struct frame *f, int *coords)
           if (vtbl != NULL)
             {
               drop_target->hwnd = hwnd;
+              drop_target->ref_count = 0;
               drop_target->i_drop_target.lpVtbl = vtbl;
               vtbl->QueryInterface = w32_drop_target_QueryInterface;
               vtbl->AddRef = w32_drop_target_AddRef;