diff --git a/fb/fbgc.c b/fb/fbgc.c
index 4af8f55f4..a9a46ae83 100644
--- a/fb/fbgc.c
+++ b/fb/fbgc.c
@@ -152,7 +152,24 @@ fbValidateGC(GCPtr pGC, unsigned long changes, DrawablePtr pDrawable)
         pPriv->fg = pGC->fgPixel & mask;
         pPriv->bg = pGC->bgPixel & mask;
 
+        /*
+         * Widening the planemask to cover the padding bits lets the blit and
+         * fill paths take their "all ones" shortcuts, but it also makes them
+         * write those bits. A rootless server keeps the alpha channel of its
+         * on screen windows opaque by clearing the alpha bits from the
+         * planemask (see miext/rootless/rootlessGC.c), and it can only do so
+         * from its own ValidateGC. mi revalidates the GC in the middle of a
+         * drawing operation for dashed lines, dashed arcs and opaque text,
+         * and by then the rootless funcs have been unwrapped by the damage
+         * layer, so that reassertion never happens. Leave the padding bits
+         * alone when they aren't part of the depth.
+         */
+#ifdef ROOTLESS_SAFEALPHA
+        if (pDrawable->depth == pDrawable->bitsPerPixel &&
+            (pGC->planemask & depthMask) == depthMask)
+#else
         if ((pGC->planemask & depthMask) == depthMask)
+#endif
             pPriv->pm = mask;
         else
             pPriv->pm = pGC->planemask & mask;