schcom: prove two clients and stacking order

The compositor already walked a surface table, but nothing had ever run
two clients at once, so "it composites" rested on a single rectangle.

schcom-test now starts two overlapping clients and checks the overlap
region from the host: the later surface has to win there. A compositor
that cannot order two surfaces cannot host a window manager, and that is
the property SchWM will be built on.

Two bugs in the test itself, both of which would have read as compositor
faults:

  - it waited for the first "committed" line and screenshotted then, so
    the second client had not drawn yet. It waits for both now.
  - one probe point sat at 400,300 -- dead centre of an 800x600 screen,
    which is exactly where the pointer is drawn. Moved clear of it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

projects/schcom/README.md

@@ -16,7 +16,9 @@ and Schcom exposes a protocol for it rather than implementing policy itself.
 commits; the result is composited and presented. Verified end to end by
 `make schcom-test`, which boots SchOS, runs the compositor and a client, then
 screendumps the framebuffer and asserts the pixels **from the host** — the
-compositor's own report that it composited something proves nothing.
+compositor's own report that it composited something proves nothing. It runs
+two overlapping clients and checks that the newer one wins in the overlap: a
+compositor that cannot order two surfaces cannot host a window manager.
 
 What exists:
 
@@ -26,6 +28,7 @@ What exists:
 - surface / attach / **commit** semantics: nothing is shown mid-draw
 - input from `/dev/input/eventN`, routed to the topmost surface
 - clients that die are cleaned up, including their shm segments
+- **multiple clients**, composited bottom-to-top in creation order
 
 What does not exist yet: damage tracking beyond "repaint everything", multiple
 outputs, subsurfaces, and any focus model beyond "topmost". Those are additions

tools/posix/schcom_test.sh

@@ -25,8 +25,9 @@ EOF
 
 (
   sleep 26
-  printf 'schcom --frames 8 &\n';                                    sleep 6
+  printf 'schcom --frames 24 &\n';                                                  sleep 6
-  printf 'schcom-demo --colour 0x00C04020 --at 100 80 --size 240 160 --wait 25\n'; sleep 14
+  printf 'schcom-demo --colour 0x00C04020 --at 100 80 --size 240 160 --wait 40 &\n'; sleep 10
+  printf 'schcom-demo --colour 0x002060C0 --at 260 180 --size 240 160 --wait 30 &\n'; sleep 12
   printf '\n'
   sleep 40
 ) | timeout -k 5 120 qemu-system-x86_64 \
@@ -38,9 +39,11 @@ EOF
       >/dev/null 2>&1 &
 QPID=$!
 
-# Wait until the client says it committed, then photograph.
+# Wait until *both* clients have committed, then photograph. Breaking on the
-for i in $(seq 1 60); do
+# first one screenshots a desktop the second client has not reached yet, which
-  grep -aq 'committed' "$LOG" 2>/dev/null && break
+# looks exactly like a compositor that cannot handle two clients.
+for i in $(seq 1 90); do
+  [ "$(grep -ac 'committed' "$LOG" 2>/dev/null)" -ge 2 ] && break
   sleep 1
 done
 sleep 2
@@ -106,12 +109,30 @@ if near(desktop, (0x20, 0x30, 0x40)):
 else:
     print(f"  FAIL: 20,20 is {desktop}, expected (32, 48, 64)"); fails += 1
 
-border = at(100, 80)                       # top-left of the surface: white border
+border = at(100, 80)                       # top-left of the first surface
 if near(border, (0xFF, 0xFF, 0xFF)):
     print(f"  [ok] border pixel at 100,80 is {border} -- surface landed exactly")
 else:
     print(f"  FAIL: 100,80 is {border}, expected white border"); fails += 1
 
+# Second client, created later, so it is above the first where they overlap.
+second = at(460, 320)                      # inside surface 2, clear of the pointer
+if near(second, (0x20, 0x60, 0xC0)):
+    print(f"  [ok] second surface at 460,320 is {second} -- two clients composited")
+else:
+    print(f"  FAIL: 460,320 is {second}, expected (32, 96, 192)"); fails += 1
+
+# 260,180..340,240 is where the two overlap. The later surface must win: if
+# stacking were ignored the first client's colour would show through here, and
+# a compositor that cannot order two windows cannot host a window manager.
+overlap = at(300, 220)
+if near(overlap, (0x20, 0x60, 0xC0)):
+    print(f"  [ok] overlap at 300,220 is {overlap} -- newer surface on top")
+elif near(overlap, (0xC0, 0x40, 0x20)):
+    print(f"  FAIL: overlap shows the older surface -- stacking order ignored"); fails += 1
+else:
+    print(f"  FAIL: overlap at 300,220 is {overlap}, expected the newer surface"); fails += 1
+
 print("schcom end-to-end: PASSED" if not fails else f"schcom end-to-end: {fails} FAILED")
 sys.exit(1 if fails else 0)
 EOF