On first run, try to guess a good default icon size based on screen DPI

Otherwise icons are miniscule when loading QGIS on hidpi screens, and
users must know that they need to manually change the icon size
to make it usable...

src/app/qgisapp.cpp

@@ -50,6 +50,7 @@
 #include <QProgressDialog>
 #include <QRegExp>
 #include <QRegExpValidator>
+#include <QScreen>
 #include <QShortcut>
 #include <QSpinBox>
 #include <QSplashScreen>
@@ -1033,8 +1034,20 @@ QgisApp::QgisApp( QSplashScreen *splash, bool restorePlugins, bool skipVersionCh
   }
 
   // Set icon size of toolbars
-  int size = settings.value( QStringLiteral( "IconSize" ), QGIS_ICON_SIZE ).toInt();
+  if ( settings.contains( QStringLiteral( "IconSize" ) ) )
-  setIconSizes( size );
+  {
+    int size = settings.value( QStringLiteral( "IconSize" ) ).toInt();
+    if ( size < 16 )
+      size = QGIS_ICON_SIZE;
+    setIconSizes( size );
+  }
+  else
+  {
+    // first run, guess a good icon size
+    int size = chooseReasonableDefaultIconSize();
+    settings.setValue( QStringLiteral( "IconSize" ), size );
+    setIconSizes( size );
+  }
 
   mSplash->showMessage( tr( "Initializing file filters" ), Qt::AlignHCenter | Qt::AlignBottom );
   qApp->processEvents();
@@ -1729,6 +1742,31 @@ QgsCoordinateReferenceSystem QgisApp::defaultCrsForNewLayers() const
   return defaultCrs;
 }
 
+int QgisApp::chooseReasonableDefaultIconSize() const
+{
+  QScreen *screen = QApplication::screens().at( 0 );
+  if ( screen->physicalDotsPerInch() < 115 )
+  {
+    // no hidpi screen, use default size
+    return QGIS_ICON_SIZE;
+  }
+  else
+  {
+    double size = fontMetrics().width( QStringLiteral( "XXX" ) );
+    if ( size < 24 )
+      return 16;
+    else if ( size < 32 )
+      return 24;
+    else if ( size < 48 )
+      return 32;
+    else if ( size < 64 )
+      return 48;
+    else
+      return 64;
+  }
+
+}
+
 void QgisApp::readSettings()
 {
   QgsSettings settings;

src/app/qgisapp.h

@@ -1688,6 +1688,9 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
 
     QgsCoordinateReferenceSystem defaultCrsForNewLayers() const;
 
+    //! Attempts to choose a reasonable default icon size based on the window's screen DPI
+    int chooseReasonableDefaultIconSize() const;
+
     QgisAppStyleSheet *mStyleSheetBuilder = nullptr;
 
     // actions for menus and toolbars -----------------