X11: Add Xdbe support to message boxes (thanks, Melker!).

Without this, message boxes with a lot of text will noticibly flicker as
you mouse over buttons.

Fixes Bugzilla #2343.

src/video/x11/SDL_x11dyn.h

@@ -50,6 +50,9 @@
 #if SDL_VIDEO_DRIVER_X11_XCURSOR
 #include <X11/Xcursor/Xcursor.h>
 #endif
+#if SDL_VIDEO_DRIVER_X11_XDBE
+#include <X11/extensions/Xdbe.h>
+#endif
 #if SDL_VIDEO_DRIVER_X11_XINERAMA
 #include <X11/extensions/Xinerama.h>
 #endif

src/video/x11/SDL_x11messagebox.c

@@ -83,6 +83,10 @@ typedef struct SDL_MessageBoxDataX11
     Display *display;
     int screen;
     Window window;
+#if SDL_VIDEO_DRIVER_X11_XDBE
+    XdbeBackBuffer buf;
+    SDL_bool xdbe;                      /* Whether Xdbe is present or not */
+#endif
     long event_mask;
     Atom wm_protocols;
     Atom wm_delete_message;
@@ -347,6 +351,12 @@ X11_MessageBoxShutdown( SDL_MessageBoxDataX11 *data )
         data->font_struct = NULL;
     }
 
+#if SDL_VIDEO_DRIVER_X11_XDBE
+    if ( SDL_X11_HAVE_XDBE && data->xdbe ) {
+        X11_XdbeDeallocateBackBufferName(data->display, data->buf);
+    }
+#endif
+
     if ( data->display ) {
         if ( data->window != None ) {
             X11_XWithdrawWindow( data->display, data->window, data->screen );
@@ -445,6 +455,20 @@ X11_MessageBoxCreateWindow( SDL_MessageBoxDataX11 *data )
     }
 
     X11_XMapRaised( display, data->window );
+
+#if SDL_VIDEO_DRIVER_X11_XDBE
+    /* Initialise a back buffer for double buffering */
+    if (SDL_X11_HAVE_XDBE) {
+        int xdbe_major, xdbe_minor;
+        if (X11_XdbeQueryExtension(display, &xdbe_major, &xdbe_minor) != 0) {
+            data->xdbe = SDL_TRUE;
+            data->buf = X11_XdbeAllocateBackBufferName(display, data->window, XdbeUndefined);
+        } else {
+            data->xdbe = SDL_FALSE;
+        }
+    }
+#endif
+
     return 0;
 }
 
@@ -453,9 +477,16 @@ static void
 X11_MessageBoxDraw( SDL_MessageBoxDataX11 *data, GC ctx )
 {
     int i;
-    Window window = data->window;
+    Drawable window = data->window;
     Display *display = data->display;
 
+#if SDL_VIDEO_DRIVER_X11_XDBE
+    if (SDL_X11_HAVE_XDBE && data->xdbe) {
+        window = data->buf;
+        X11_XdbeBeginIdiom(data->display);
+    }
+#endif
+
     X11_XSetForeground( display, ctx, data->color[ SDL_MESSAGEBOX_COLOR_BACKGROUND ] );
     X11_XFillRectangle( display, window, ctx, 0, 0, data->dialog_width, data->dialog_height );
 
@@ -505,6 +536,16 @@ X11_MessageBoxDraw( SDL_MessageBoxDataX11 *data, GC ctx )
                          buttondata->text, buttondatax11->length );
         }
     }
+
+#if SDL_VIDEO_DRIVER_X11_XDBE
+    if (SDL_X11_HAVE_XDBE && data->xdbe) {
+        XdbeSwapInfo swap_info;
+        swap_info.swap_window = data->window;
+        swap_info.swap_action = XdbeUndefined;
+        X11_XdbeSwapBuffers(data->display, &swap_info, 1);
+        X11_XdbeEndIdiom(data->display);
+    }
+#endif
 }
 
 /* Loop and handle message box event messages until something kills it. */
@@ -568,7 +609,7 @@ X11_MessageBoxLoop( SDL_MessageBoxDataX11 *data )
         case MotionNotify:
             if ( has_focus ) {
                 /* Mouse moved... */
-                int previndex = data->mouse_over_index;
+                const int previndex = data->mouse_over_index;
                 data->mouse_over_index = GetHitButtonIndex( data, e.xbutton.x, e.xbutton.y );
                 if (data->mouse_over_index == previndex) {
                     draw = SDL_FALSE;

src/video/x11/SDL_x11sym.h

@@ -230,6 +230,20 @@ SDL_X11_SYM(void,XcursorImageDestroy,(XcursorImage *a),(a),)
 SDL_X11_SYM(Cursor,XcursorImageLoadCursor,(Display *a,const XcursorImage *b),(a,b),return)
 #endif
 
+/* Xdbe support */
+#if SDL_VIDEO_DRIVER_X11_XDBE
+SDL_X11_MODULE(XDBE)
+SDL_X11_SYM(Status,XdbeQueryExtension,(Display *dpy,int *major_version_return,int *minor_version_return),(dpy,major_version_return,minor_version_return),return)
+SDL_X11_SYM(XdbeBackBuffer,XdbeAllocateBackBufferName,(Display *dpy,Window window,XdbeSwapAction swap_action),(dpy,window,swap_action),return)
+SDL_X11_SYM(Status,XdbeDeallocateBackBufferName,(Display *dpy,XdbeBackBuffer buffer),(dpy,buffer),return)
+SDL_X11_SYM(Status,XdbeSwapBuffers,(Display *dpy,XdbeSwapInfo *swap_info,int num_windows),(dpy,swap_info,num_windows),return)
+SDL_X11_SYM(Status,XdbeBeginIdiom,(Display *dpy),(dpy),return)
+SDL_X11_SYM(Status,XdbeEndIdiom,(Display *dpy),(dpy),return)
+SDL_X11_SYM(XdbeScreenVisualInfo*,XdbeGetVisualInfo,(Display *dpy,Drawable *screen_specifiers,int *num_screens),(dpy,screen_specifiers,num_screens),return)
+SDL_X11_SYM(void,XdbeFreeVisualInfo,(XdbeScreenVisualInfo *visual_info),(visual_info),)
+SDL_X11_SYM(XdbeBackBufferAttributes*,XdbeGetBackBufferAttributes,(Display *dpy,XdbeBackBuffer buffer),(dpy,buffer),return)
+#endif
+
 /* Xinerama support */
 #if SDL_VIDEO_DRIVER_X11_XINERAMA
 SDL_X11_MODULE(XINERAMA)

src/video/x11/SDL_x11video.h

@@ -34,6 +34,9 @@
 #if SDL_VIDEO_DRIVER_X11_XCURSOR
 #include <X11/Xcursor/Xcursor.h>
 #endif
+#if SDL_VIDEO_DRIVER_X11_XDBE
+#include <X11/extensions/Xdbe.h>
+#endif
 #if SDL_VIDEO_DRIVER_X11_XINERAMA
 #include <X11/extensions/Xinerama.h>
 #endif