Rebuild Displays around the canvas, and let the transaction keep color

Claude-Session: https://claude.ai/code/session_01Ms2FbjQy31TVf3CEvQhGM8

tests/quickshell/display-arrangement-contract

@@ -31,8 +31,106 @@ for contract in \
 done
 rg -Fq 'DisplayArrangement {' "$page" \
     || fail 'Displays page does not expose the arrangement canvas'
-rg -Fq 'visible: Displays.monitors.length > 1' "$page" \
-    || fail 'arrangement is shown for a single display'
+
+# Flipped by the displays redesign, deliberately and in this direction.
+#
+# The canvas used to be hidden below two displays. That left a laptop opening
+# its Displays page on a picker offering a list of one, and made the page's
+# most legible surface the one thing a single-display machine never saw. It is
+# the hero of the page now: a solo display is rendered in it, and only dragging
+# goes away, because there is nothing to arrange that display against.
+# The bindings of one QML element, found by matching its braces. A fixed-size
+# window around the element would either miss a binding or catch a neighbour's,
+# and both of those are the wrong answer here: the page's selector chips and its
+# Workspaces card are legitimately gated on having two displays.
+element_binding() { # file, element, pattern, present|absent
+    python3 - "$@" <<'PY'
+import re
+import sys
+
+path, opener, pattern, expectation = sys.argv[1:5]
+text = open(path, encoding="utf-8").read()
+start = text.find(opener)
+if start < 0:
+    raise SystemExit(1)
+depth = 0
+end = len(text)
+for index in range(start + len(opener) - 1, len(text)):
+    if text[index] == "{":
+        depth += 1
+    elif text[index] == "}":
+        depth -= 1
+        if depth == 0:
+            end = index
+            break
+found = re.search(pattern, text[start:end]) is not None
+raise SystemExit(0 if found == (expectation == "present") else 1)
+PY
+}
+
+python3 - "$page" <<'PY' \
+    || fail 'the arrangement canvas is gated on a monitor count again -- on the canvas itself or on the card holding it -- so a single display sees no canvas'
+import re
+import sys
+
+text = open(sys.argv[1], encoding="utf-8").read()
+start = text.find("DisplayArrangement {")
+if start < 0:
+    raise SystemExit(1)
+
+
+def own_bindings(open_brace, stop):
+    """The block's own bindings, with nested elements left out: a sibling
+    card's gate is its business, and this is only about the canvas."""
+    kept = []
+    depth = 0
+    for char in text[open_brace + 1:stop]:
+        if char == "{":
+            depth += 1
+        elif char == "}":
+            depth -= 1
+            if depth < 0:
+                break
+        elif depth == 0:
+            kept.append(char)
+    return "".join(kept)
+
+
+# The canvas's own bindings, and those of whatever element holds it: a gate on
+# either one is a canvas a single display never sees. This lived on the card
+# rather than on the canvas before the redesign, which is exactly why the
+# element alone is not enough to look at.
+stack = []
+for index, char in enumerate(text[:start]):
+    if char == "{":
+        stack.append(index)
+    elif char == "}" and stack:
+        stack.pop()
+if not stack:
+    raise SystemExit(1)
+
+regions = [own_bindings(start + len("DisplayArrangement {") - 1, len(text)),
+           own_bindings(stack[-1], start)]
+gate = re.compile(r"visible:.*monitors\.length")
+raise SystemExit(1 if any(gate.search(region) for region in regions) else 0)
+PY
+
+for contract in \
+    'One display connected' \
+    'draggable'; do
+    rg -Fq "$contract" "$component" \
+        || fail "a solo display is not explained or not protected from dragging: $contract"
+done
+element_binding "$component" 'DragHandler {' 'enabled:' present \
+    || fail 'the drag handler is unconditional, so a solo display can be dragged around a canvas with nothing to arrange against'
+
+# Mirroring has no position of its own: the compositor stacks a mirrored output
+# on its target. The canvas says which display it is mirroring rather than
+# drawing it wherever its stale coordinates happen to point.
+rg -Fq 'Mirrors ' "$component" \
+    || fail 'a mirrored display is drawn with no badge saying what it mirrors'
+rg -Fq 'mirrorOf' "$component" \
+    || fail 'the canvas does not read the mirror flag off its rects'
 [[ -f "$identify" ]] || fail 'DisplayIdentify.qml is missing'
 for contract in \
     'model: Quickshell.screens' \
@@ -94,6 +192,17 @@ keyboard="$(ipc keyboardFixture)"
 jq -e '.afterArrow == 2990 and .afterShiftArrow == 2890' <<<"$keyboard" >/dev/null \
     || fail "keyboard movement did not use 10/100 logical-pixel steps: $keyboard"
 
+solo="$(ipc soloFixture)"
+jq -e '(.rects | length) == 1 and .scale > 0 and .draggable == false' <<<"$solo" >/dev/null \
+    || fail "a single display did not render solo with dragging disabled: $solo"
+
+mirror="$(ipc mirrorFixture)"
+jq -e '(.rects | length) == 2
+    and (.rects[1].mirrorOf == "DP-2")
+    and (.rects[1].x == .rects[0].x) and (.rects[1].y == .rects[0].y)' \
+    <<<"$mirror" >/dev/null \
+    || fail "a mirrored display was not stacked on its target and flagged: $mirror"
+
 primary="$(ipc primaryFixture)"
 jq -e '. == [
     {"name":"DP-2","x":-3000,"y":0,"primary":false},