CNA-BUG-179: Win32 EnterExclusiveFullscreen fails silently yet GetFullscreenMode reports ExclusiveFullscreen

CNA snapshot 009d40f5  ·  Known Issues › Current bugs  ·  source links pinned to 009d40f5

EnterExclusiveFullscreen returns silently when a monitor-info, EnumDisplaySettingsW or ChangeDisplaySettingsExW step fails, though its comment says a refusal is reported; SetFullscreenMode still records ExclusiveFullscreen, so GetFullscreenMode misreports it.

Identifier

CNA-BUG-179

Category

Bug

Subsystem

Platforms

Status

Open

Verified against

CNA 009d40f5 (009d40f5dd085c4e674d3479675fac84b12b3e0a)

Severity

Low (a triage suggestion, not a project priority)

Evidence basis

Source-verified: read at TARGET, not executed

Tests touching this area

Yes: see Current tests

Affected contract

CNA::Platform::Win32::Win32Window::SetFullscreenMode(WindowFullscreenMode::ExclusiveFullscreen) and GetFullscreenMode()

Expected behaviour

EnterExclusiveFullscreen's comment says the display-mode change “is attempted for the window's own monitor, and a refusal is reported rather than silently downgraded into the borderless mode the caller did not ask for.” So a failed exclusive-mode change should surface, and GetFullscreenMode() should not claim exclusive fullscreen the window is not in.

Actual behaviour at TARGET

EnterExclusiveFullscreen first runs EnterBorderlessFullscreen, then returns silently if MonitorFromWindow/GetMonitorInfoW or EnumDisplaySettingsW fails, and sets exclusiveModeChanged_ only when ChangeDisplaySettingsExW returns DISP_CHANGE_SUCCESSFUL. On any of those failures nothing is reported and the window is left in borderless fullscreen, but SetFullscreenMode unconditionally sets fullscreenMode_ = ExclusiveFullscreen, so GetFullscreenMode() returns ExclusiveFullscreen. (The mode change also requests the monitor's current resolution, so exclusive fullscreen never changes the resolution.)

Source locations

• modules/platform/src/Win32/Win32Window.cpp — EnterExclusiveFullscreen (silent early returns; exclusiveModeChanged_) and SetFullscreenMode (sets fullscreenMode_ regardless)

Evidence

Checked by reading EnterExclusiveFullscreen, LeaveFullscreen and SetFullscreenMode at 009d40f5. Not executed: exclusive fullscreen needs a real Windows display (Wine reports a fixed DPI and one monitor). The Development Win32 page records the comment/behaviour drift.

Focused reproduction

No focused reproduction is known. Nothing has been invented here; the evidence above is what exists.

Current tests

Win32FullscreenStateTests.cpp and Win32WindowTests.cpp's fullscreen cases exercise borderless fullscreen and the windowed-state snapshot; no test drives exclusive fullscreen or a failed display-mode change.

Regression test

On a real Windows host (or a display abstraction), force ChangeDisplaySettingsExW to fail and require that either SetFullscreenMode reports the refusal or GetFullscreenMode() reports the mode actually in effect.

Blast radius

Windows games that request exclusive fullscreen on a monitor where the mode change fails; they run in borderless fullscreen while GetFullscreenMode() misreports exclusive.

Workaround

Treat borderless and exclusive fullscreen as interchangeable on Win32; do not rely on GetFullscreenMode() to confirm an exclusive-mode change.