Steps to reproduce:

• Launch Windows Magnifier (Windows+PavNumPlus)
• Alt+Tab to Magnifier window
• Press "Display" button and select "Full screen" option
• Press "Options" button and check the "Invert colors" checkbox
• Open Vision panel in NVDA settings
• Uncheck the "Always show a warning" checkbox (probably does not matter but I did this way)
• Check the "Enable screen curtain" checkbox

Actual behavior:

• Screen curtain is not enabled
• an error beep is heard: cf. logWinMagnifier+ScreenCurtain.txt
• a message box indicating"Could not load the Screen Curtain vision enhancement provider" appears.

Expected behavior:

• Best choice: screen curtain is enabled and the screen becomes black
• Mid choice: screen curtain is enabled, but the screen is white due to color inversion of the magnifier
• Minimal choice: screen curtain cannot be activated, no error appears in the log and the message explains to the user why screen curtain cannot be activated

Other tests performed

• Test 2: Windows Magnifier with normal colors + screen curtain: no error, the screen becomes black
• Test 3: Zoomtext 11 (normal colors) + screen curtain: no error, the screen becomes black
• Test 4: Zoomtext 11 (inverted colors) + screen curtain: no error, the screen becomes white
• Test 5:
  • Start Windows Magnifier with normal colors
  • Activate screen curtain: no error, screen black
  • Invert Windows Magnifier colors: no error, the screen remains black (so this seems possible...)

System configuration

NVDA installed/portable/running from source:

From source

NVDA version:

Version : source-master-c750926
(last version)

Windows version:

Windows 10 1709

Name and version of other software in use when reproducing the issue:

For test 1: none.
For some other tests: Zoomtext 11

Other information about your system:

Other questions

Does the issue still occur after restarting your PC?

Yes

Have you tried any other versions of NVDA? If so, please report their behaviors.

No other stable version.
I have already seen this issue few time after the screen curtain was merged into alpha but did not bother to fill an issue at the time. I do not remember however the version of the alpha in which I have already seen the issue.