Add Linux background drag and held-button tools

.github/workflows/nix-build.yml

@@ -55,6 +55,17 @@ jobs:
             visual: true
             result_link: result-linux-background-terminal-gif
             artifact_name: cua-driver-linux-background-terminal-gif
+          - name: Linux parallel multi-cursor drag test
+            check_attr: cua-driver-linux-parallel-drag-xserver
+            timeout_minutes: 20
+            visual: true
+            result_link: result-linux-parallel-drag-xserver
+            artifact_name: cua-driver-linux-parallel-drag-xserver
+          # NOTE: the older "parallel multi-cursor drag GIF" scenario
+          # (linux-parallel-drag-gif.nix) is intentionally not in this matrix —
+          # it hand-launches Xorg, which can't get a VT/seat in the emulated
+          # nixos-test VM. The services.xserver entry above supersedes it; the
+          # old scenario file is kept for local/real-X runs.
           # Full entries (CDP / Tk focus-free-write overrides) — kept as-is.
           - name: Linux background GUI test (chromium)
             check_attr: cua-driver-linux-background-gui-chromium
@@ -278,6 +289,7 @@ jobs:
             const artifactNames = [
               'cua-driver-linux-cursor-click-gif',
               'cua-driver-linux-background-terminal-gif',
+              'cua-driver-linux-parallel-drag-xserver',
               'cua-driver-linux-background-gui-chromium',
               'cua-driver-linux-background-gui-tk',
               'cua-driver-linux-background-gui-gtk3-gedit',

flake.nix

@@ -79,6 +79,29 @@
                   services.cua-driver.package = cuaDriverPackage;
                 };
               };
+
+              # Multi-cursor (MPX) parallel-drag test on a REAL Xorg brought up
+              # by NixOS services.xserver (dummy video + libinput, on a seat via
+              # a display manager). This is the CI-viable replacement for the
+              # hand-launched-Xorg linux-parallel-drag-gif.nix (which timed out
+              # because a self-launched Xorg couldn't get a VT/seat in the
+              # emulated nixos-test VM). Proves uinput slaves enumerate as X
+              # devices, two cursors draw concurrent window-targeted events, and
+              # the shield grab keeps focus off the drag.
+              cua-driver-linux-parallel-drag-xserver = import ./nix/cua-driver/tests/linux-parallel-drag-xserver.nix {
+                inherit pkgs;
+                inherit (pkgs) lib;
+                cuaDriverModule = {
+                  imports = [ ./nix/cua-driver/module.nix ];
+                  services.cua-driver.package = cuaDriverPackage;
+                };
+              };
+
+              # NOTE: cua-driver-linux-parallel-drag-gif (nix/cua-driver/tests/
+              # linux-parallel-drag-gif.nix) is intentionally NOT a flake check —
+              # it hand-launches Xorg, which can't get a VT/seat in the emulated
+              # GHA nixos-test VM. It is superseded by the services.xserver test
+              # above and kept only for local/real-X manual runs.
             }
             // pkgs.lib.optionalAttrs (system == "x86_64-linux") (
               # Background GUI input coverage — one independent matrix job per