Very strange, I've never seen that issue before. Would you mind posting your log file? Are you running with a laptop trackpad or a mouse?
I was initially using a laptop trackpad, then following your message I tried plugging a USB mouse into my laptop. It works with the mouse, not with the trackpad.
Here is the output:
Playscii v0.9.12
Loading config from /home/myname/.config/Playscii/playscii.cfg...
Config loaded.
Detecting hardware...
CPU: x86_64
OS: Linux-5.2.2-arch1-1-ARCH-x86_64-with-arch
Python: 3.7.3 (default, Jun 24 2019, 04:54:02) [GCC 9.1.0] (64bit)
Modules: PySDL2: 0.9.6, numpy: 1.16.4, PyOpenGL: 3.1.0, appdirs: 1.4.3, PIL: 6.1.0
SDL: 2.0.6 hg-12373:8feb5da6f2fb, SDLmixer: 2.0.0
Detected screen resolution: 1280 x 800, window: 1024 x 640
GPU: Intel Open Source Technology Center - Mesa DRI Intel(R) Ivybridge Mobile
OpenGL detected: 3.0 Mesa 19.1.3
GLSL detected: 1.30
Vertex Array Object support found.
Maximum supported texture size: 16384 x 16384
Loaded key binds from /home/myname/.config/Playscii/binds.cfg
init done.
Thank you for using Playscii! <3
Are you invoking the picker by pressing the space bar, or holding it? Both should be valid and you can toggle that behavior in the Tool menu, but I've noticed some laptops lock out the trackpad when typing is happening (eg a key is being held). If you open a text editor and hold spacebar, does the trackpad work? If not it might be a hardware/OS level issue. Let me know if the behavior only happens with Playscii; it might be due to a quirk of SDL2 for example.