I have investigated the black screen with only mouse pointer showing up after installation. The problem is that sddm is crashing. The journal logs show that OpenGL isn’t able to function and/or is conflicting with QT.
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-helper[418]: pam_unix(sddm-greeter:session): session opened for user sddm(uid=969) by (uid=0)
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: High-DPI autoscaling not Enabled
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: Reading from "/usr/share/wayland-sessions/plasmawayland.desktop"
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: Reading from "/usr/share/xsessions/plasma.desktop"
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: Loading theme configuration from "qrc:/theme/theme.conf"
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: Connected to the daemon.
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: QFont::fromString: Invalid description '(empty)'
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: QGLXContext: Failed to create dummy context
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: Loading qrc:/theme/Main.qml...
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: file:///usr/lib/qt/qml/SddmComponents/LayoutBox.qml:35:5: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: qrc:/theme/Main.qml:41:5: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: Adding view for "Virtual-1" QRect(0,0 1664x936)
Apr 12 14:05:26 tester-standardpcq35ich92009 sddm-greeter[428]: Failed to create OpenGL context for format QSurfaceFormat(version 2.0, options QFlags<QSurfaceFormat::FormatOption>(ResetNotification), depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize -1, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 1, colorSpace QSurfaceFormat::DefaultColorSpace, profile QSurfaceFormat::NoProfile)
Apr 12 14:05:27 tester-standardpcq35ich92009 systemd-coredump[439]: Process 428 (sddm-greeter) of user 969 dumped core.
#18 0x00005583d2d5c80f main (sddm-greeter + 0x1880f)
#21 0x00005583d2d5e065 _start (sddm-greeter + 0x1a065)
░░ Subject: Process 428 (sddm-greeter) dumped core
░░ Process 428 (sddm-greeter) crashed and dumped core.
Apr 12 14:05:27 tester-standardpcq35ich92009 sddm-helper[418]: pam_unix(sddm-greeter:session): session closed for user sddm
This issue seems to only affect new installs.
A workaround/solution is to open a terminal and remove the libvdpau and nvidia-340xx-utils:
sudo pacman -Rdd libvdpau
sudo pacman -R nvidia-340xx-utils
You can then go ahead and reinstall libvdpau if you need to.
If you can’t open a Terminal (ctrl-alt-F3) example using a VM, you can edit Grub command (e - edit), put a 3 at the end of the linux line and save it (ctrl-x) and it will take you to command prompt bypassing graphical target and be in multiuser target (non-gui). You can login and then run the removal commands.