Query UI font from system on Windows

Previously, we determined the UI font from a predefined set of fonts
each mapped to a language. This works well if the user doesn't change
their Windows UI font, but if one does, they will get the default UI
font associated with their language instead of the one the set.

This commit replaces ProgSettings::getFontName with
ProgSettings::getUIFont, which uses the SystemParametersInfo API to
query the message font from the system, which will allow users to have a
custom font. It will also not interfere with different languages, as the
message font will be appropriately set by default there.

src/qt/qt_main.cpp

@@ -622,7 +622,7 @@ main(int argc, char *argv[])
     fprintf(stderr, "Qt: version %s, platform \"%s\"\n", qVersion(), QApplication::platformName().toUtf8().data());
     ProgSettings::loadTranslators(&app);
 #ifdef Q_OS_WINDOWS
-    QApplication::setFont(QFont(ProgSettings::getFontName(lang_id), 9));
+    QApplication::setFont(ProgSettings::getUIFont());
     SetCurrentProcessExplicitAppUserModelID(L"86Box.86Box");
 #endif
 

src/qt/qt_mainwindow.cpp

@@ -2431,7 +2431,7 @@ MainWindow::changeEvent(QEvent *event)
 #ifdef Q_OS_WINDOWS
     if (event->type() == QEvent::LanguageChange) {
         auto size = this->centralWidget()->size();
-        QApplication::setFont(QFont(ProgSettings::getFontName(lang_id), 9));
+        QApplication::setFont(ProgSettings::getUIFont());
         QApplication::processEvents();
         main_window->centralWidget()->setFixedSize(size);
         QApplication::processEvents();

src/qt/qt_progsettings.cpp

@@ -28,6 +28,8 @@
 #ifdef Q_OS_WINDOWS
 #    include <QSysInfo>
 #    include <QVersionNumber>
+#    define  WIN32_LEAN_AND_MEAN
+#    include <windows.h>
 #endif
 
 extern "C" {
@@ -158,26 +160,42 @@ ProgSettings::~ProgSettings()
 }
 
 #ifdef Q_OS_WINDOWS
-/* Return the standard font name on Windows, which is overridden per-language
-   to prevent CJK fonts with embedded bitmaps being chosen as a fallback. */
-QString
-ProgSettings::getFontName(int langId)
+/* Returns the standard UI font for Windows, which by default varies for different
+   languages. It can also be changed via external tools, if the user wants that.
+
+   We use the message font here since that is what most Windows components and
+   other third-party programs use. */
+QFont
+ProgSettings::getUIFont()
 {
-    QString langCode = languageIdToCode(lang_id);
-    if (langCode == "ja-JP") {
-        /* Check for Windows 10 or later to choose the appropriate system font */
-        if (QVersionNumber::fromString(QSysInfo::kernelVersion()).majorVersion() >= 10)
-            return "Yu Gothic UI";
-        else
-            return "Meiryo UI";
-    } else if (langCode == "ko-KR")
-        return "Malgun Gothic";
-    else if (langCode == "zh-CN")
-        return "Microsoft YaHei";
-    else if (langCode == "zh-TW")
-        return "Microsoft JhengHei";
-    else
-        return "Segoe UI";
+    // Get the system (primary monitor) DPI. The font returned by
+    // SystemParametersInfo is scaled according to this and we need
+    // to get the font size in points to pass into QFont's constructor.
+    HDC hdc = GetDC(NULL);
+    int systemDpi = GetDeviceCaps(hdc, LOGPIXELSY);
+    ReleaseDC(NULL, hdc);
+
+    // Get the font metrics.
+    NONCLIENTMETRICSW ncm = {};
+    ncm.cbSize = sizeof(ncm);
+    // This should never happen, but just to be safe, return Segoe UI if
+    // SPI fails.
+    if (!SystemParametersInfoW(SPI_GETNONCLIENTMETRICS, sizeof(ncm), &ncm, 0))
+    {
+        return QFont("Segoe UI", 9);
+    }
+
+    QString fontName = QString::fromWCharArray(ncm.lfMessageFont.lfFaceName);
+    // Windows' conversion from points to pixels goes as follows:
+    // 
+    //     -MulDiv(PointSize, GetDeviceCaps(hDC, LOGPIXELSY), 72)
+    // 
+    // (source: https://learn.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-createfontw)
+    //
+    // Let's reverse that calculation to get the point size from the message font.
+    int fontSize = -MulDiv(ncm.lfMessageFont.lfHeight, 72, systemDpi);
+    
+    return QFont(fontName, fontSize);
 }
 #endif
 

src/qt/qt_progsettings.hpp

@@ -15,7 +15,7 @@ public:
     explicit ProgSettings(QWidget *parent = nullptr);
     ~ProgSettings();
 #ifdef Q_OS_WINDOWS
-    static QString getFontName(int langId);
+    static QFont getUIFont();
 #endif
     static int     languageCodeToId(QString langCode);
     static QString languageIdToCode(int id);

src/qt/qt_vmmanager_details.cpp

@@ -165,7 +165,7 @@ VMManagerDetails::VMManagerDetails(QWidget *parent)
     connect(this, &VMManagerDetails::styleUpdated, portsSection, &VMManagerDetailSection::updateStyle);
     connect(this, &VMManagerDetails::styleUpdated, otherSection, &VMManagerDetailSection::updateStyle);
 
-    QApplication::setFont(QFont(ProgSettings::getFontName(lang_id), 9));
+    QApplication::setFont(ProgSettings::getUIFont());
 #endif
 
     sysconfig = new VMManagerSystem();

src/qt/qt_vmmanager_mainwindow.cpp

@@ -269,7 +269,7 @@ VMManagerMainWindow::changeEvent(QEvent *event)
 {
 #ifdef Q_OS_WINDOWS
     if (event->type() == QEvent::LanguageChange) {
-        QApplication::setFont(QFont(ProgSettings::getFontName(lang_id), 9));
+        QApplication::setFont(QFont(ProgSettings::getUIFont()));
     }
 #endif
     QWidget::changeEvent(event);