diff --git a/TASKS.md b/TASKS.md
index 7806e93..8e5459e 100644
--- a/TASKS.md
+++ b/TASKS.md
@@ -51,7 +51,7 @@ Progress truth. Top to bottom, one at a time. A task is ticked only after its co
 
 - [x] T19 macOS and Windows: `Blurred` or `Transparent` or `Opaque` from settings, applied live.
 - [x] T20 Wayland: `ext_background_effect_v1` on the window's surface.
-- [ ] T21 X11: `_KDE_NET_WM_BLUR_BEHIND_REGION` on the window.
+- [x] T21 X11: `_KDE_NET_WM_BLUR_BEHIND_REGION` on the window.
 
 ## 12. Menus and shortcuts
 
diff --git a/tasks/reviews.md b/tasks/reviews.md
index ded7b68..c63741b 100644
--- a/tasks/reviews.md
+++ b/tasks/reviews.md
@@ -103,3 +103,7 @@ Each task gets at most three codex rounds. Each round: the commit, the verdict,
 ## T20
 
 - Round 1, 6523632 and 107069b: PASS. 6523632 fixed a test the first Linux run caught: on Linux a program that ignores SIGHUP reads EIO when its pty closes, so GNU cat exits 1 where macOS gives EOF.
+
+## T21
+
+- Round 1, b5e4967: PASS. The work found that Ely's ResizeBorder painted a client-drawn window's pane solid, so no Linux window showed the desktop and T20's blur could not show; Ely 5dc4dd1 adds ResizeBorder::glass, and T20's region now covers the pane alone.