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

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

tests/quickshell/display-layout-contract

@@ -54,4 +54,34 @@ jq -e '.valid == true
 invalid="$(qs_for_harness ipc call display-layout-test invalid)"
 jq -e 'all(. == false)' <<<"$invalid" >/dev/null || fail "an invalid layout was accepted: $invalid"
 
+# Mirroring is the one arrangement where the position on file is not the
+# position on the desk. Hyprland stacks a mirrored output on its target and
+# ignores whatever the rule asked for, so the geometry here has to agree:
+# a mirrored display contributes nothing to the desktop's bounds, and the
+# canvas draws it on top of what it mirrors rather than at stale coordinates.
+mirror="$(qs_for_harness ipc call display-layout-test mirror)"
+#
+# The mirrored record keeps the coordinates it was stored with -- normalize
+# shifts everything by the primary's anchor, and applying that to a position
+# nothing reads back would be arithmetic for its own sake. It contributes
+# nothing to the bounds, so the desktop measures 3000 x 2000: one display.
+jq -e '.valid == true
+    and .normalized == [
+      {"name":"DP-2","x":0,"y":0,"primary":true,"mirrorOf":""},
+      {"name":"HDMI-A-1","x":3140,"y":80,"primary":false,"mirrorOf":"DP-2"}]
+    and .bounds == {"x":0,"y":0,"width":3000,"height":2000}
+    and (.rects | length) == 2
+    and .rects[1].mirrorOf == "DP-2" and .rects[1].mirrored == true
+    and .rects[0].mirrored == false
+    and .rects[1].x == .rects[0].x and .rects[1].y == .rects[0].y
+    and .rects[1].width == .rects[0].width' \
+    <<<"$mirror" >/dev/null || fail "mirrored geometry was wrong: $mirror"
+
+# A mirror that names itself, an absent output, or a display that is itself
+# mirroring is not a layout with a cosmetic problem -- it is one the compositor
+# will answer in a way nobody predicted.
+invalid_mirrors="$(qs_for_harness ipc call display-layout-test invalidMirrors)"
+jq -e 'all(. == false)' <<<"$invalid_mirrors" >/dev/null \
+    || fail "an impossible mirror was accepted: $invalid_mirrors"
+
 printf 'display layout contract: PASS\n'