Some drag'and'drop improvements.

First: disable d'n'd events by default; most apps don't need these at all, and
if an app doesn't explicitly handle these, each drop on the window will cause
a memory leak if the events are enabled. This follows the guidelines we have
for SDL_TEXTINPUT events already.

Second: when events are enabled or disabled, signal the video layer, as it
might be able to inform the OS, causing UI changes or optimizations (for
example, dropping a file icon on a Cocoa app that isn't accepting drops will
cause macOS to show a rejection animation instead of the drop operation just
vanishing into the ether, X11 might show a different cursor when dragging
onto an accepting window, etc).

Third: fill in the drop event details in the test library and enable the
events in testwm.c for making sure this all works as expected.

src/video/SDL_sysvideo.h

@@ -304,6 +304,9 @@ struct SDL_VideoDevice
     /* Hit-testing */
     int (*SetWindowHitTest)(SDL_Window * window, SDL_bool enabled);
 
+    /* Tell window that app enabled drag'n'drop events */
+    void (*AcceptDragAndDrop)(SDL_Window * window, SDL_bool accept);
+
     /* * * */
     /* Data common to all drivers */
     SDL_bool is_dummy;
@@ -454,6 +457,8 @@ extern void SDL_OnApplicationDidEnterBackground(void);
 extern void SDL_OnApplicationWillEnterForeground(void);
 extern void SDL_OnApplicationDidBecomeActive(void);
 
+extern void SDL_ToggleDragAndDropSupport(void);
+
 #endif /* SDL_sysvideo_h_ */
 
 /* vi: set ts=4 sw=4 expandtab: */

src/video/SDL_video.c

@@ -1331,9 +1331,43 @@ SDL_UpdateFullscreenMode(SDL_Window * window, SDL_bool fullscreen)
 #define CREATE_FLAGS \
     (SDL_WINDOW_OPENGL | SDL_WINDOW_BORDERLESS | SDL_WINDOW_RESIZABLE | SDL_WINDOW_ALLOW_HIGHDPI | SDL_WINDOW_ALWAYS_ON_TOP | SDL_WINDOW_SKIP_TASKBAR | SDL_WINDOW_POPUP_MENU | SDL_WINDOW_UTILITY | SDL_WINDOW_TOOLTIP | SDL_WINDOW_VULKAN | SDL_WINDOW_MINIMIZED)
 
+static SDL_INLINE SDL_bool
+IsAcceptingDragAndDrop(void)
+{
+    if ((SDL_GetEventState(SDL_DROPFILE) == SDL_ENABLE) ||
+        (SDL_GetEventState(SDL_DROPTEXT) == SDL_ENABLE)) {
+        return SDL_TRUE;
+    }
+    return SDL_FALSE;
+}
+
+/* prepare a newly-created window */
+static SDL_INLINE void
+PrepareDragAndDropSupport(SDL_Window *window)
+{
+    if (_this->AcceptDragAndDrop) {
+        _this->AcceptDragAndDrop(window, IsAcceptingDragAndDrop());
+    }
+}
+
+/* toggle d'n'd for all existing windows. */
+void
+SDL_ToggleDragAndDropSupport(void)
+{
+    if (_this && _this->AcceptDragAndDrop) {
+        const SDL_bool enable = IsAcceptingDragAndDrop();
+        SDL_Window *window;
+        for (window = _this->windows; window; window = window->next) {
+            _this->AcceptDragAndDrop(window, enable);
+        }
+    }
+}
+
 static void
 SDL_FinishWindowCreation(SDL_Window *window, Uint32 flags)
 {
+    PrepareDragAndDropSupport(window);
+
     if (flags & SDL_WINDOW_MAXIMIZED) {
         SDL_MaximizeWindow(window);
     }
@@ -1552,6 +1586,9 @@ SDL_CreateWindowFrom(const void *data)
         SDL_DestroyWindow(window);
         return NULL;
     }
+
+    PrepareDragAndDropSupport(window);
+
     return window;
 }
 

src/video/cocoa/SDL_cocoavideo.m

@@ -105,6 +105,7 @@ Cocoa_CreateDevice(int devindex)
     device->DestroyWindow = Cocoa_DestroyWindow;
     device->GetWindowWMInfo = Cocoa_GetWindowWMInfo;
     device->SetWindowHitTest = Cocoa_SetWindowHitTest;
+    device->AcceptDragAndDrop = Cocoa_AcceptDragAndDrop;
 
     device->shape_driver.CreateShaper = Cocoa_CreateShaper;
     device->shape_driver.SetWindowShape = Cocoa_SetWindowShape;

src/video/cocoa/SDL_cocoawindow.h

@@ -148,6 +148,7 @@ extern void Cocoa_SetWindowGrab(_THIS, SDL_Window * window, SDL_bool grabbed);
 extern void Cocoa_DestroyWindow(_THIS, SDL_Window * window);
 extern SDL_bool Cocoa_GetWindowWMInfo(_THIS, SDL_Window * window, struct SDL_SysWMinfo *info);
 extern int Cocoa_SetWindowHitTest(SDL_Window *window, SDL_bool enabled);
+extern void Cocoa_AcceptDragAndDrop(SDL_Window * window, SDL_bool accept);
 
 #endif /* SDL_cocoawindow_h_ */
 

src/video/cocoa/SDL_cocoawindow.m

@@ -1354,9 +1354,6 @@ Cocoa_CreateWindow(_THIS, SDL_Window * window)
     [nswindow setContentView:contentView];
     [contentView release];
 
-    /* Allow files and folders to be dragged onto the window by users */
-    [nswindow registerForDraggedTypes:[NSArray arrayWithObject:(NSString *)kUTTypeFileURL]];
-
     if (SetupWindowData(_this, window, nswindow, SDL_TRUE) < 0) {
         [nswindow release];
         return -1;
@@ -1864,6 +1861,17 @@ Cocoa_SetWindowHitTest(SDL_Window * window, SDL_bool enabled)
     return 0;  /* just succeed, the real work is done elsewhere. */
 }
 
+void
+Cocoa_AcceptDragAndDrop(SDL_Window * window, SDL_bool accept)
+{
+    SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
+    if (accept) {
+        [data->nswindow registerForDraggedTypes:[NSArray arrayWithObject:(NSString *)kUTTypeFileURL]];
+    } else {
+        [data->nswindow unregisterDraggedTypes];
+    }
+}
+
 int
 Cocoa_SetWindowOpacity(_THIS, SDL_Window * window, float opacity)
 {

src/video/windows/SDL_windowsvideo.c

@@ -164,6 +164,7 @@ WIN_CreateDevice(int devindex)
     device->DestroyWindowFramebuffer = WIN_DestroyWindowFramebuffer;
     device->OnWindowEnter = WIN_OnWindowEnter;
     device->SetWindowHitTest = WIN_SetWindowHitTest;
+    device->AcceptDragAndDrop = WIN_AcceptDragAndDrop;
 
     device->shape_driver.CreateShaper = Win32_CreateShaper;
     device->shape_driver.SetWindowShape = Win32_SetWindowShape;

src/video/windows/SDL_windowswindow.c

@@ -289,9 +289,6 @@ SetupWindowData(_THIS, SDL_Window * window, HWND hwnd, HWND parent, SDL_bool cre
         videodata->RegisterTouchWindow(hwnd, (TWF_FINETOUCH|TWF_WANTPALM));
     }
 
-    /* Enable dropping files */
-    DragAcceptFiles(hwnd, TRUE);
-
     data->initializing = SDL_FALSE;
 
     /* All done! */
@@ -979,6 +976,13 @@ WIN_SetWindowOpacity(_THIS, SDL_Window * window, float opacity)
     return 0;
 }
 
+void
+WIN_AcceptDragAndDrop(SDL_Window * window, SDL_bool accept)
+{
+    const SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
+    DragAcceptFiles(data->hwnd, accept ? TRUE : FALSE);
+}
+
 #endif /* SDL_VIDEO_DRIVER_WINDOWS */
 
 /* vi: set ts=4 sw=4 expandtab: */

src/video/windows/SDL_windowswindow.h

@@ -78,6 +78,7 @@ extern SDL_bool WIN_GetWindowWMInfo(_THIS, SDL_Window * window,
 extern void WIN_OnWindowEnter(_THIS, SDL_Window * window);
 extern void WIN_UpdateClipCursor(SDL_Window *window);
 extern int WIN_SetWindowHitTest(SDL_Window *window, SDL_bool enabled);
+extern void WIN_AcceptDragAndDrop(SDL_Window * window, SDL_bool accept);
 
 #endif /* SDL_windowswindow_h_ */
 

src/video/x11/SDL_x11video.c

@@ -260,6 +260,7 @@ X11_CreateDevice(int devindex)
     device->DestroyWindowFramebuffer = X11_DestroyWindowFramebuffer;
     device->GetWindowWMInfo = X11_GetWindowWMInfo;
     device->SetWindowHitTest = X11_SetWindowHitTest;
+    device->AcceptDragAndDrop = X11_AcceptDragAndDrop;
 
     device->shape_driver.CreateShaper = X11_CreateShaper;
     device->shape_driver.SetWindowShape = X11_SetWindowShape;

src/video/x11/SDL_x11window.c

@@ -390,7 +390,6 @@ X11_CreateWindow(_THIS, SDL_Window * window)
     const char *wintype_name = NULL;
     long compositor = 1;
     Atom _NET_WM_PID;
-    Atom XdndAware, xdnd_version = 5;
     long fevent = 0;
 
 #if SDL_VIDEO_OPENGL_GLX || SDL_VIDEO_OPENGL_EGL
@@ -651,11 +650,6 @@ X11_CreateWindow(_THIS, SDL_Window * window)
                  PropertyChangeMask | StructureNotifyMask |
                  KeymapStateMask | fevent));
 
-    XdndAware = X11_XInternAtom(display, "XdndAware", False);
-    X11_XChangeProperty(display, w, XdndAware, XA_ATOM, 32,
-                 PropModeReplace,
-                 (unsigned char*)&xdnd_version, 1);
-
     X11_XFlush(display);
 
     return 0;
@@ -1604,6 +1598,22 @@ X11_SetWindowHitTest(SDL_Window *window, SDL_bool enabled)
     return 0;  /* just succeed, the real work is done elsewhere. */
 }
 
+void
+X11_AcceptDragAndDrop(SDL_Window * window, SDL_bool accept)
+{
+    SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
+    Display *display = data->videodata->display;
+    Atom XdndAware = X11_XInternAtom(display, "XdndAware", False);
+
+    if (accept) {
+        Atom xdnd_version = 5;
+        X11_XChangeProperty(display, data->xwindow, XdndAware, XA_ATOM, 32,
+                     PropModeReplace, (unsigned char*)&xdnd_version, 1);
+    } else {
+        X11_XDeleteProperty(display, data->xwindow, XdndAware);
+    }
+}
+
 #endif /* SDL_VIDEO_DRIVER_X11 */
 
 /* vi: set ts=4 sw=4 expandtab: */

src/video/x11/SDL_x11window.h

@@ -104,6 +104,7 @@ extern void X11_DestroyWindow(_THIS, SDL_Window * window);
 extern SDL_bool X11_GetWindowWMInfo(_THIS, SDL_Window * window,
                                     struct SDL_SysWMinfo *info);
 extern int X11_SetWindowHitTest(SDL_Window *window, SDL_bool enabled);
+extern void X11_AcceptDragAndDrop(SDL_Window * window, SDL_bool accept);
 
 #endif /* SDL_x11window_h_ */