Preferred monitor: chosen in Optionen > Allgemein, used at the next start

The Internet-Optionen frame becomes Bildschirm with a Monitor dropdown
(Hauptmonitor or a connected monitor, kept by device path in
[display] Monitor=). The borderless window and the render size use that
monitor, falling back to the main monitor when it is not connected.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/mods/monitor.cpp

@@ -0,0 +1,160 @@
+// Preferred monitor (request of 2026-10-25): a setting in the graphics options
+// picks the monitor the game starts on; with none set, or the preferred one
+// not connected, the system's main monitor.
+//
+// The game window and its render size are decided when the display mode is
+// set at startup (borderless.cpp, widescreen.cpp ApplyRenderSize): both ask
+// GameMonitorRect. A change applies at the next start.
+//
+// Monitors are told apart by their device path (DisplayConfigGetDeviceInfo,
+// target name: "\\?\DISPLAY#GSM5B7F#..."), which survives restarts, monitor
+// order changes and other ports' changes, unlike the GDI names \\.\DISPLAYn.
+//
+// sacredK.ini:
+//   [display]
+//   Monitor=<device path>   ; empty: the main monitor
+
+#include "mods/monitor.h"
+
+#include <algorithm>
+
+#include "re/config.h"
+#include "re/log.h"
+
+namespace mods::monitor {
+namespace {
+
+constexpr const char* kSection = "display";
+constexpr const char* kKey = "Monitor";
+
+std::string Utf8(const wchar_t* text) {
+    const int size = WideCharToMultiByte(CP_UTF8, 0, text, -1, nullptr, 0, nullptr, nullptr);
+    if (size <= 1) {
+        return {};
+    }
+    std::string result(static_cast<size_t>(size - 1), '\0');
+    WideCharToMultiByte(CP_UTF8, 0, text, -1, result.data(), size, nullptr, nullptr);
+    return result;
+}
+
+struct Target {
+    std::wstring gdiName;  // \\.\DISPLAYn of the source showing it
+    std::string id;
+    std::wstring name;
+};
+
+// The active display paths: which monitor shows which GDI display.
+std::vector<Target> Targets() {
+    UINT32 pathCount = 0;
+    UINT32 modeCount = 0;
+    std::vector<DISPLAYCONFIG_PATH_INFO> paths;
+    std::vector<DISPLAYCONFIG_MODE_INFO> modes;
+    LONG result;
+    do {
+        if (GetDisplayConfigBufferSizes(QDC_ONLY_ACTIVE_PATHS, &pathCount, &modeCount) != ERROR_SUCCESS) {
+            return {};
+        }
+        paths.resize(pathCount);
+        modes.resize(modeCount);
+        result = QueryDisplayConfig(QDC_ONLY_ACTIVE_PATHS, &pathCount, paths.data(), &modeCount, modes.data(),
+                                    nullptr);
+    } while (result == ERROR_INSUFFICIENT_BUFFER);
+    if (result != ERROR_SUCCESS) {
+        return {};
+    }
+    paths.resize(pathCount);
+    std::vector<Target> targets;
+    for (const DISPLAYCONFIG_PATH_INFO& path : paths) {
+        DISPLAYCONFIG_SOURCE_DEVICE_NAME source = {};
+        source.header.type = DISPLAYCONFIG_DEVICE_INFO_GET_SOURCE_NAME;
+        source.header.size = sizeof(source);
+        source.header.adapterId = path.sourceInfo.adapterId;
+        source.header.id = path.sourceInfo.id;
+        DISPLAYCONFIG_TARGET_DEVICE_NAME target = {};
+        target.header.type = DISPLAYCONFIG_DEVICE_INFO_GET_TARGET_NAME;
+        target.header.size = sizeof(target);
+        target.header.adapterId = path.targetInfo.adapterId;
+        target.header.id = path.targetInfo.id;
+        if (DisplayConfigGetDeviceInfo(&source.header) != ERROR_SUCCESS ||
+            DisplayConfigGetDeviceInfo(&target.header) != ERROR_SUCCESS) {
+            continue;
+        }
+        targets.push_back({source.viewGdiDeviceName, Utf8(target.monitorDevicePath), target.monitorFriendlyDeviceName});
+    }
+    return targets;
+}
+
+BOOL CALLBACK AddMonitor(HMONITOR handle, HDC, LPRECT, LPARAM data) {
+    auto* gdi = reinterpret_cast<std::vector<MONITORINFOEXW>*>(data);
+    MONITORINFOEXW info = {};
+    info.cbSize = sizeof(info);
+    if (GetMonitorInfoW(handle, &info)) {
+        gdi->push_back(info);
+    }
+    return TRUE;
+}
+
+}  // namespace
+
+std::vector<Monitor> List() {
+    std::vector<MONITORINFOEXW> gdi;
+    EnumDisplayMonitors(nullptr, nullptr, &AddMonitor, reinterpret_cast<LPARAM>(&gdi));
+    const std::vector<Target> targets = Targets();
+    std::vector<Monitor> monitors;
+    for (const MONITORINFOEXW& info : gdi) {
+        Monitor monitor{"", L"", info.rcMonitor, (info.dwFlags & MONITORINFOF_PRIMARY) != 0};
+        for (const Target& target : targets) {
+            if (target.gdiName == info.szDevice) {  // mirrored displays: the first monitor
+                monitor.id = target.id;
+                monitor.name = target.name;
+                break;
+            }
+        }
+        if (monitor.id.empty()) {
+            monitor.id = Utf8(info.szDevice);  // no display config (remote desktop, ...)
+        }
+        if (monitor.name.empty()) {
+            monitor.name = L"Monitor " + std::to_wstring(monitors.size() + 1);
+        }
+        monitors.push_back(monitor);
+    }
+    std::stable_sort(monitors.begin(), monitors.end(),
+                     [](const Monitor& a, const Monitor& b) { return a.primary && !b.primary; });
+    return monitors;
+}
+
+std::string PreferredId() {
+    return re::ConfigString(kSection, kKey, "");
+}
+
+void SetPreferredId(const std::string& id) {
+    re::SetConfigString(kSection, kKey, id.c_str());
+}
+
+RECT GameMonitorRect() {
+    const std::vector<Monitor> monitors = List();
+    const std::string preferred = PreferredId();
+    const Monitor* chosen = nullptr;
+    for (const Monitor& monitor : monitors) {
+        if ((!preferred.empty() && monitor.id == preferred) || (!chosen && monitor.primary)) {
+            chosen = &monitor;
+        }
+    }
+    if (!chosen) {
+        // No monitor reported: the main monitor's rectangle as Windows has it.
+        MONITORINFO info = {sizeof(info)};
+        GetMonitorInfoW(MonitorFromPoint({0, 0}, MONITOR_DEFAULTTOPRIMARY), &info);
+        return info.rcMonitor;
+    }
+    const bool found = !preferred.empty() && chosen->id == preferred;
+    static bool logged = false;
+    if (!logged) {
+        logged = true;
+        re::Log("monitor: %ls %ldx%ld at (%ld,%ld)%s", chosen->name.c_str(), chosen->rect.right - chosen->rect.left,
+                chosen->rect.bottom - chosen->rect.top, chosen->rect.left, chosen->rect.top,
+                found ? "" : preferred.empty() ? ", main monitor" : ", preferred monitor not connected: main monitor");
+    }
+    return chosen->rect;
+}
+
+}  // namespace mods::monitor