[Bug 59652] New: ole32: DoDragDrop does not fall back to WM_DROPFILES for legacy applications

[WineHQ Bugzilla]

http://bugs.winehq.org/show_bug.cgi?id=59652

            Bug ID: 59652
           Summary: ole32: DoDragDrop does not fall back to WM_DROPFILES
                    for legacy applications
           Product: Wine
           Version: 11.6
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ole32
          Assignee: [e-mail]
          Reporter: [e-mail]
      Distribution: ---

In Microsoft Windows, when an OLE Drag & Drop source application (using
DoDragDrop) drops data onto a target application that does not implement
IDropTarget but has the WS_EX_ACCEPTFILES style, the ole32.dll library
automatically acts as a bridge. It extracts CF_HDROP data, allocates memory in
the target process, and posts a WM_DROPFILES message.

Currently, Wine's ole32.dll does not implement this fallback mechanism. If
IDropTarget is not found on the target window, Wine simply returns
DROPEFFECT_NONE and aborts the operation, even if a drop was requested
(DRAGDROP_S_DROP). This breaks drag-and-drop functionality for many legacy
applications that rely on WM_DROPFILES.

Steps to Reproduce:
    Run an OLE-aware application that can act as a drag source (e.g., Total
Commander, modern file explorers).
    Run a legacy application that accepts files via WM_DROPFILES but does not
implement OLE IDropTarget (e.g., Notepad++, etc.).
    Drag a file from the source application to the target application.

Actual Results: The drop fails. DoDragDrop returns DROPEFFECT_NONE.

Expected Results: The file should be successfully dropped and opened by the
target application, as it happens on Windows.

Patch: The following patch implements the WM_DROPFILES fallback bridge in
dlls/ole32/ole2.c. It correctly identifies legacy targets, marshals the
CF_HDROP data into the target process, and posts the WM_DROPFILES message. This
fixes the issue in both XWayland and native Wayland backend.

diff

--- a/dlls/ole32/ole2.c
+++ b/dlls/ole32/ole2.c
@@ -2246,6 +2246,90 @@
     return target;
 }

+/* WM_DROPFILES bridge for legacy apps without IDropTarget */
+typedef struct {
+    DWORD pFiles;
+    POINT pt;
+    BOOL  fNC;
+    BOOL  fWide;
+} WINE_DROPFILES;
+
+static HWND find_drop_target(POINT pt)
+{
+    HWND walk = GetTopWindow(GetDesktopWindow());
+    DWORD our_pid = GetCurrentProcessId();
+
+    while (walk) {
+        if (IsWindowVisible(walk)) {
+            RECT rc;
+            DWORD wpid = 0;
+            GetWindowThreadProcessId(walk, &wpid);
+            if (wpid && wpid != our_pid) {
+                GetWindowRect(walk, &rc);
+                if (PtInRect(&rc, pt)) {
+                    HWND target = walk;
+                    while (target) {
+                        if (GetWindowLongW(target, GWL_EXSTYLE) &
WS_EX_ACCEPTFILES)
+                            return target;
+                        target = GetParent(target);
+                    }
+                    return walk;
+                }
+            }
+        }
+        walk = GetWindow(walk, GW_HWNDNEXT);
+    }
+    return NULL;
+}
+
+static BOOL bridge_wm_dropfiles(IDataObject *dataObject, HWND target, POINT
pt)
+{
+    FORMATETC fmt = { CF_HDROP, NULL, DVASPECT_CONTENT, -1, TYMED_HGLOBAL };
+    STGMEDIUM stg;
+    WINE_DROPFILES *df;
+    DWORD target_pid;
+    HANDLE hProc;
+    SIZE_T size;
+    void *remote;
+
+    if (FAILED(IDataObject_GetData(dataObject, &fmt, &stg)))
+        return FALSE;
+    if (stg.tymed != TYMED_HGLOBAL || !stg.hGlobal) {
+        ReleaseStgMedium(&stg);
+        return FALSE;
+    }
+
+    df = GlobalLock(stg.hGlobal);
+    if (!df) {
+        ReleaseStgMedium(&stg);
+        return FALSE;
+    }
+    size = GlobalSize(stg.hGlobal);
+    df->pt = pt;
+    df->fNC = FALSE;
+
+    GetWindowThreadProcessId(target, &target_pid);
+    hProc = OpenProcess(PROCESS_VM_OPERATION | PROCESS_VM_WRITE, FALSE,
target_pid);
+    if (!hProc) {
+        GlobalUnlock(stg.hGlobal);
+        ReleaseStgMedium(&stg);
+        return FALSE;
+    }
+
+    remote = VirtualAllocEx(hProc, NULL, size, MEM_COMMIT, PAGE_READWRITE);
+    if (remote) {
+        WriteProcessMemory(hProc, remote, df, size, NULL);
+        PostMessageW(target, WM_DROPFILES, (WPARAM)remote, 0);
+    }
+
+    CloseHandle(hProc);
+    GlobalUnlock(stg.hGlobal);
+    ReleaseStgMedium(&stg);
+    return remote != NULL;
+}
+
 static void drag_end( TrackerWindowInfo *info )
 {
     HRESULT hr;
@@ -2279,6 +2363,17 @@
     else
         *info->pdwEffect = DROPEFFECT_NONE;

+    /* Bridge OLE drop to WM_DROPFILES for legacy apps */
+    if (*info->pdwEffect == DROPEFFECT_NONE && info->returnValue ==
DRAGDROP_S_DROP)
+    {
+        POINT pt;
+        HWND target;
+        pt.x = info->curMousePos.x;
+        pt.y = info->curMousePos.y;
+        target = find_drop_target(pt);
+        if (target && bridge_wm_dropfiles(info->dataObject, target, pt))
+            *info->pdwEffect = DROPEFFECT_COPY;
+    }
 }

 static HRESULT give_feedback( TrackerWindowInfo *info )

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