fix: don't scale UI if user overrides scaling mode

Historically users could make PoB render at 100% scale on a machine that
had higher display scaling factors by overriding the compatibility
settings on the executable to indicate that DPI scaling should be done
by "Application".

As we're now DPI-aware the scaling setting does no longer enable that
user intent as scaling is always done by the application. We can however
still manually read out the override intent from the AppCompat registry
keys to honor the user's old intent.

This change checks both the machine and user registry keys for overrides
and honors them in that order, if any.

Author: zao

engine/system/win/sys_video.cpp

@@ -51,6 +51,7 @@ class sys_video_c : public sys_IVideo {
 	sys_main_c* sys = nullptr;
 
 	bool	initialised = false;
+	bool	ignoreDpiScale = false;
 	GLFWwindow* wnd = nullptr;
 
 	void	RefreshMonitorInfo();
@@ -278,6 +279,63 @@ struct sys_programIcons_c {
 	std::deque<std::vector<uint8_t>> imageDatas;
 };
 
+bool ShouldIgnoreDpiScale() {
+#ifdef _WIN32
+	std::wstring const appChoice = L"HIGHDPIAWARE", sysChoice = L"DPIUNAWARE", enhanceFlag = L"GDIDPISCALING";
+	std::wstring const subKey = LR"(Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers)";
+
+	std::vector<wchar_t> progStr(1 << 16);
+	GetModuleFileNameW(NULL, progStr.data(), progStr.size());
+	std::filesystem::path progPath(progStr.data());
+	progPath = std::filesystem::canonical(progPath);
+
+	auto considerKey = [&](HKEY rootKey) -> std::optional<bool> {
+		std::vector<wchar_t> valData(1 << 16);
+		DWORD valType{}, valSize = valData.size() / 2;
+		LRESULT res = RegGetValueW(rootKey, subKey.c_str(), progPath.wstring().c_str(), RRF_RT_REG_SZ, &valType, valData.data(), &valSize);
+		if (res == ERROR_SUCCESS) {
+			std::wstring val = valData.data();
+			if (wcsstr(val.c_str(), appChoice.c_str())) {
+				return true;
+			}
+			else if (wcsstr(val.c_str(), sysChoice.c_str())) {
+				return false;
+			}
+		}
+		return {};
+		};
+
+	struct ScopedRegKey {
+		HKEY key{};
+
+		ScopedRegKey& operator = (ScopedRegKey const&) = delete;
+		ScopedRegKey(ScopedRegKey const&) = delete;
+		~ScopedRegKey() {
+			if (key) { RegCloseKey(key); }
+		}
+
+		HKEY* operator & () { return &key; }
+		operator HKEY () const { return key; }
+	};
+
+	// Prioritize the global setting over the user setting to follow Windows' semantics.
+	ScopedRegKey hklm{};
+	if (RegOpenKeyExA(HKEY_LOCAL_MACHINE, nullptr, 0, KEY_READ, &hklm) == ERROR_SUCCESS) {
+		if (auto ignore = considerKey(hklm)) {
+			return *ignore;
+		}
+	}
+
+	ScopedRegKey hkcu{};
+	if (RegOpenCurrentUser(KEY_READ, &hkcu) == ERROR_SUCCESS) {
+		if (auto ignore = considerKey(hkcu)) {
+			return *ignore;
+		}
+	}
+#endif
+	return false;
+}
+
 // ==================
 // System Video Class
 // ==================
@@ -383,6 +441,7 @@ int sys_video_c::Apply(sys_vidSet_s* set)
 		}
 	}
 	else {
+		ignoreDpiScale = ShouldIgnoreDpiScale();
 		glfwWindowHint(GLFW_RESIZABLE, !!(cur.flags & VID_RESIZABLE));
 		glfwWindowHint(GLFW_VISIBLE, GLFW_FALSE); // Start hidden to not flash the user with a stock window.
 		glfwWindowHint(GLFW_MAXIMIZED, GLFW_FALSE); // Start restored in order to position the window before maximizing.
@@ -551,7 +610,10 @@ int sys_video_c::Apply(sys_vidSet_s* set)
 			});
 		glfwSetWindowContentScaleCallback(wnd, [](GLFWwindow* wnd, float xScale, float yScale) {
 			auto sys = (sys_main_c*)glfwGetWindowUserPointer(wnd);
-			sys->video->vid.dpiScale = xScale;
+			auto video = (sys_video_c*)sys->video;
+			if (!video->ignoreDpiScale) {
+				video->vid.dpiScale = xScale;
+			}
 			});
 
 		// Adjust window look and position
@@ -576,7 +638,9 @@ int sys_video_c::Apply(sys_vidSet_s* set)
 
 	glfwGetFramebufferSize(wnd, &vid.fbSize[0], &vid.fbSize[1]);
 	glfwGetWindowSize(wnd, &vid.size[0], &vid.size[1]);
-	glfwGetWindowContentScale(wnd, &sys->video->vid.dpiScale, nullptr);
+	if (!ignoreDpiScale) {
+		glfwGetWindowContentScale(wnd, &sys->video->vid.dpiScale, nullptr);
+	}
 
 	initialised = true;
 	return 0;