diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index fa81b2e953..df6f07496e 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -2499,9 +2499,9 @@ Face Attributes
 @code{nil} to not use this face for the space between the end of the
 line and the edge of the window.  When Emacs merges several faces for
 displaying the empty space beyond end of line, only those faces with
-@code{:extend} non-@code{nil} will be merged.  By default, only
-@code{region} and @code{hl-line} faces have this attribute set to
-@code{t}.
+@code{:extend} non-@code{nil} will be merged.  This attribute is
+different from the others in that when a theme doesn't define it for a
+face, the value from the default spec is inherited.
 
 @end table
 
diff --git a/etc/NEWS b/etc/NEWS
index 28bcb720cd..3d0781c24c 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -437,15 +437,16 @@ to 'completion-styles' or 'completion-category-overrides' to use it.
 The new face attribute ':extend' controls whether to use the face for
 displaying the empty space beyond end of line (EOL) till the edge of
 the window.  By default, this attribute is non-nil only for 'region',
-'secondary-selection', 'hl-line' and some faces of Diff and Ediff
-modes; any other face that crosses end of line will not affect the
-display of the empty space at EOL.  This is to make Emacs behave more
-like other GUI applications with respect to displaying faces that
-cross line boundaries.
-
-Themes that redefine faces should add a non-nil ':extend' attribute to
-the above-mentioned faces, to keep the behavior of the default face
-definitions.
+'secondary-selection', 'hl-line' and some faces of Diff, Ediff,
+LogView and SMerge modes; any other face that crosses end of line will
+not affect the display of the empty space at EOL.  This is to make
+Emacs behave more like other GUI applications with respect to
+displaying faces that cross line boundaries.
+
+This attribute behaves specially when theme definitions are applied:
+if the theme doesn't specify its value for a face, the value from the
+default spec is used.  Consequently, themes usually shouldn't touch
+this attribute at all.
 
 ** Connection-local variables
 
diff --git a/lisp/faces.el b/lisp/faces.el
index dc5bcca760..0f31628f5f 100644
--- a/lisp/faces.el
+++ b/lisp/faces.el
@@ -1669,7 +1669,7 @@ face-spec-recalc
   ;; `theme-face' records.
   (let ((theme-faces (get face 'theme-face))
 	(no-match-found 0)
-	face-attrs theme-face-applied)
+        default-attrs face-attrs theme-face-applied)
     (if theme-faces
 	(dolist (elt (reverse theme-faces))
 	  (setq face-attrs (face-spec-choose (cadr elt) frame no-match-found))
@@ -1677,13 +1677,20 @@ face-spec-recalc
 	    (face-spec-set-2 face frame face-attrs)
 	    (setq theme-face-applied t))))
     ;; If there was a spec applicable to FRAME, that overrides the
-    ;; defface spec entirely (rather than inheriting from it).  If
-    ;; there was no spec applicable to FRAME, apply the defface spec
-    ;; as well as any applicable X resources.
+    ;; defface spec entirely rather than inheriting from it, with the
+    ;; exception of the :extend attribute (which is inherited).
+    ;;
+    ;; If there was no spec applicable to FRAME, apply the defface
+    ;; spec as well as any applicable X resources.
+    (setq default-attrs (face-spec-choose (face-default-spec face) frame))
     (unless theme-face-applied
-      (setq face-attrs (face-spec-choose (face-default-spec face) frame))
-      (face-spec-set-2 face frame face-attrs)
+      (face-spec-set-2 face frame default-attrs)
       (make-face-x-resource-internal face frame))
+    (when (and theme-face-applied
+               (eq 'unspecified (face-attribute face :extend frame t)))
+      (let ((tail (plist-member default-attrs :extend)))
+        (and tail (face-spec-set-2 face frame
+                                   (list :extend (cadr tail))))))
     (setq face-attrs (face-spec-choose (get face 'face-override-spec) frame))
     (face-spec-set-2 face frame face-attrs)))