Skip root pixmap access when alpha == 255

When alpha is 255 (fully opaque), the pseudo-transparency blend factor
is zero (0xFFFF - 257 * 255 = 0), making the root pixmap composite a
visual no-op. However, drawBackground unconditionally calls
XRenderCreatePicture on the _XROOTPMAP_ID pixmap.

Wallpaper setters like feh free the old root pixmap via XKillClient
each time they cycle to a new image. If xmobar attempts
XRenderCreatePicture on the freed pixmap, the X server returns
BadDrawable and the default error handler terminates xmobar.

Guard the entire root pixmap code path with `when (alpha < 255)` so
that fully opaque bars never touch _XROOTPMAP_ID.

2 files changed, 23 insertions, 16 deletions

diff --git a/changelog.md b/changelog.md
index aeb07e3..bc56e1d 100644
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,6 @@
 ## Version 0.51 (unrelease)
 
+- Fix BadDrawable crash when wallpaper setters free the root pixmap while alpha is 255
 - base dependency relaxed to 4.21
 
 ## Version 0.50 (June, 2025)
diff --git a/src/Xmobar/X11/XRender.hsc b/src/Xmobar/X11/XRender.hsc
index 5ad0391..4e4d448 100644
--- a/src/Xmobar/X11/XRender.hsc
+++ b/src/Xmobar/X11/XRender.hsc
@@ -91,22 +91,28 @@ drawBackground d p bgc alpha (Rectangle x y wid ht) = do
       withRenderFill d
                      (XRenderColor 0 0 0 (257 * alpha))
                      (render pictOpSrc bgfill pic)
-    -- Handle transparency
-    internAtom d "_XROOTPMAP_ID" False >>= \xid ->
-      let xroot = defaultRootWindow d in
-      alloca $ \x1 ->
-      alloca $ \x2 ->
-      alloca $ \x3 ->
-      alloca $ \x4 ->
-      alloca $ \pprop -> do
-        xGetWindowProperty d xroot xid 0 1 False 20 x1 x2 x3 x4 pprop
-        prop <- peek pprop
-        when (prop /= nullPtr) $ do
-          rootbg <- peek (castPtr prop) :: IO Pixmap
-          xFree prop
-          withRenderPicture d rootbg $ \bgpic ->
-            withRenderFill d (XRenderColor 0 0 0 (0xFFFF - 257 * alpha))
-                           (render pictOpAdd bgpic pic)
+    -- Handle pseudo-transparency by compositing the root pixmap.
+    -- Skip entirely when alpha == 255 (fully opaque) since the blend
+    -- factor would be zero, making this a no-op. More importantly,
+    -- the root pixmap (_XROOTPMAP_ID) can be freed at any time by
+    -- wallpaper setters like feh (via XKillClient), causing a
+    -- BadDrawable crash in XRenderCreatePicture if we touch it.
+    when (alpha < 255) $
+      internAtom d "_XROOTPMAP_ID" False >>= \xid ->
+        let xroot = defaultRootWindow d in
+        alloca $ \x1 ->
+        alloca $ \x2 ->
+        alloca $ \x3 ->
+        alloca $ \x4 ->
+        alloca $ \pprop -> do
+          xGetWindowProperty d xroot xid 0 1 False 20 x1 x2 x3 x4 pprop
+          prop <- peek pprop
+          when (prop /= nullPtr) $ do
+            rootbg <- peek (castPtr prop) :: IO Pixmap
+            xFree prop
+            withRenderPicture d rootbg $ \bgpic ->
+              withRenderFill d (XRenderColor 0 0 0 (0xFFFF - 257 * alpha))
+                             (render pictOpAdd bgpic pic)
 
 -- | Parses color into XRender color (allocation not necessary!)
 parseRenderColor :: Display -> String -> IO XRenderColor