Weird colour glitch

I'm running on an old iMac (specs below) and got this weird glitch when applying one layer as a 'color' overlay on top of another. I worked around this and applied the colour in a different way ... but what was odd was that toggling the color layer on and off produced different pixel glitch effects ... in a boxed area that was not defined in the mask or anywhere else.

Is this because Photoshop 2019 no longer supports my GPU properly?

The 'performance' preferences pane is auto-detecting and applying ATI Radeon 5720 OpenGL Engine setting.

Should I enable 'legacy compositing' or something else to make Photoshop render stuff better on my old system?

Regards,

Peter

Adobe Photoshop Version: 20.0.0 20180920.r.24 2018/09/20: 1193433  x64

Number of Launches: 153

Operating System: Mac OS 10.13.6

System architecture: Intel CPU Family:6, Model:14, Stepping:5 with MMX, SSE Integer, SSE FP, SSE2, SSE3, SSE4.1, SSE4.2, AVX, AVX2, HyperThreading

Physical processor count: 4

Processor speed: 2800 MHz

Built-in memory: 16384 MB

Free memory: 5970 MB

Memory available to Photoshop: 14510 MB

Memory used by Photoshop: 70 %

Alias Layers: Disabled.

Modifier Palette: Disabled.

Highbeam: Enabled.

Touch Bar Property Feedback: Disabled.

Image tile size: 1024K

Image cache levels: 4

Font Preview: Medium

TextComposer: Latin

Display: 1

Main Display

Display Bounds: top=0, left=0, bottom=1440, right=2560

Display: 2

Display Bounds: top=0, left=2560, bottom=768, right=3584

OpenGL Drawing: Enabled.

OpenGL Allow Old GPUs: Not Detected.

OpenGL Drawing Mode: Advanced

OpenGL Allow Normal Mode: True.

OpenGL Allow Advanced Mode: True.

AIFCoreInitialized=1

AIFOGLInitialized=1

OGLContextCreated=1

NumGLGPUs=1

NumCLGPUs=1

NumNativeGPUs=0

glgpu[0].GLVersion="2.1"

glgpu[0].IsIntegratedGLGPU=0

glgpu[0].GLMemoryMB=1024

glgpu[0].GLName="ATI Radeon HD 5750 OpenGL Engine"

glgpu[0].GLVendor="ATI Technologies Inc."

glgpu[0].GLVendorID=4098

glgpu[0].GLRectTextureSize=16384

glgpu[0].GLRenderer="ATI Radeon HD 5750 OpenGL Engine"

glgpu[0].GLRendererID=16915202

glgpu[0].HasGLNPOTSupport=1

glgpu[0].CanCompileProgramGLSL=1

glgpu[0].GLFrameBufferOK=1

glgpu[0].glGetString[GL_SHADING_LANGUAGE_VERSION]="1.20"

glgpu[0].glGetProgramivARB[GL_FRAGMENT_PROGRAM_ARB][GL_MAX_PROGRAM_INSTRUCTIONS_ARB]=[4096]

glgpu[0].glGetIntegerv[GL_MAX_TEXTURE_UNITS]=[8]

glgpu[0].glGetIntegerv[GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS]=[16]

glgpu[0].glGetIntegerv[GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS]=[16]

glgpu[0].glGetIntegerv[GL_MAX_TEXTURE_IMAGE_UNITS]=[16]

glgpu[0].glGetIntegerv[GL_MAX_DRAW_BUFFERS]=[8]

glgpu[0].glGetIntegerv[GL_MAX_VERTEX_UNIFORM_COMPONENTS]=[3072]

glgpu[0].glGetIntegerv[GL_MAX_FRAGMENT_UNIFORM_COMPONENTS]=[3072]

glgpu[0].glGetIntegerv[GL_MAX_VARYING_FLOATS]=[128]

glgpu[0].glGetIntegerv[GL_MAX_VERTEX_ATTRIBS]=[16]

glgpu[0].extension[AIF::OGL::GL_ARB_VERTEX_PROGRAM]=1

glgpu[0].extension[AIF::OGL::GL_ARB_FRAGMENT_PROGRAM]=1

glgpu[0].extension[AIF::OGL::GL_ARB_VERTEX_SHADER]=1

glgpu[0].extension[AIF::OGL::GL_ARB_FRAGMENT_SHADER]=1

glgpu[0].extension[AIF::OGL::GL_EXT_FRAMEBUFFER_OBJECT]=1

glgpu[0].extension[AIF::OGL::GL_ARB_TEXTURE_RECTANGLE]=1

glgpu[0].extension[AIF::OGL::GL_ARB_TEXTURE_FLOAT]=1

glgpu[0].extension[AIF::OGL::GL_ARB_OCCLUSION_QUERY]=1

glgpu[0].extension[AIF::OGL::GL_ARB_VERTEX_BUFFER_OBJECT]=1

glgpu[0].extension[AIF::OGL::GL_ARB_SHADER_TEXTURE_LOD]=1

clgpu[0].CLPlatformVersion="1.2 (May 24 2018 20:07:03)"

clgpu[0].CLDeviceVersion="1.2 "

clgpu[0].IsIntegratedCLGPU=0

clgpu[0].CLMemoryMB=1024

clgpu[0].CLName="ATI Radeon HD 5750"

clgpu[0].CLVendor="AMD"

clgpu[0].CLVendorID=16915200

clgpu[0].CLDriverVersion="1.2 (Jun 29 2018 18:33:46)"

clgpu[0].CLBandwidth=3.97074e+10

clgpu[0].CLCompute=93.2039

License Type: Subscription

Serial number: 94070758929313704698

GUIDBucket:Composite Core (enable_composite_core): onComposite Core UI (comp_core_ui): offDocument Graph (enable_doc_graph): off

Application folder: /Applications/Adobe Photoshop CC 2019/

Photoshop scratch has async I/O enabled

Scratch volume(s):

  Startup, 931.3G, 391.3G free

Required Plug-ins folder: /Applications/Adobe Photoshop CC 2019/Adobe Photoshop CC 2019.app/Contents/Required/Plug-Ins/

Primary Plug-ins folder: /Applications/Adobe Photoshop CC 2019/Plug-Ins/