GetWindowDC(0) called unconditionally in __init__ prevents monitor-only usage

[mxschmitt]

Problem

MSSImplWindows.__init__() calls GetWindowDC(0) (line 207 of mss/windows/gdi.py) unconditionally during construction, even when the caller only needs monitor geometry (sct.monitors) and never calls grab().

GetWindowDC(0) can fail with WinError 5: Access is denied when:

• The desktop is temporarily inaccessible (locked screen, UAC prompt, RDP disconnect)
• GDI handles were leaked by a previous session

This means even with mss.MSS() as sct: sct.monitors fails in these scenarios, despite monitors() only needing EnumDisplayMonitors(0, ...) + GetMonitorInfoW which do not require a desktop device context.

Reproduction

import mss
# This fails with ScreenShotError("GetWindowDC: Access is denied")
# when the desktop DC is unavailable, even though we only read monitors:
with mss.MSS() as sct:
    print(sct.monitors)

Suggestion

Defer GetWindowDC(0) and CreateCompatibleDC to grab() (or a lazy init on first grab) instead of acquiring them in __init__. This would allow monitor enumeration to work regardless of desktop DC availability.

Alternatively, provide a lightweight API for monitor enumeration that doesn't construct the full capture pipeline.

Environment

• mss version: 10.2.0.dev0 (from PR Switch from GetDIBits to CreateDIBSection for screen capture on Windows #464)
• OS: Windows 11 Enterprise 10.0.26100
• Python: 3.13.13

[mxschmitt]

Additional details

mss version: Commit eced18e (post-PR #464, pre-10.2.0 release)

Exact errors observed in production (Sentry):

1. ScreenShotError: Windows graphics function failed: GetWindowDC: Access is denied. — during MSSImplWindows.__init__() at gdi.py:207
2. ScreenShotError: Windows graphics function failed: CreateDIBSection: Access is denied. — during grab() at gdi.py:363

Both occur when calling mss.MSS() solely to read sct.monitors (monitor geometry enumeration). The monitors() method only needs EnumDisplayMonitors + GetMonitorInfoW + GetSystemMetrics, none of which require a desktop DC — but __init__ unconditionally acquires one via GetWindowDC(0).

Workaround: We've replaced all mss.MSS() monitor enumeration calls with direct ctypes calls to EnumDisplayMonitors/GetMonitorInfoW (Windows) and CGGetActiveDisplayList/CGDisplayBounds (macOS) — the same APIs mss uses internally, just without the GetWindowDC(0) in the constructor.

[BoboTiG]

I'm open the an improvement in this area. Would you mind adding a regression test and open a PR to fix it?