Improve NaviCube font matching on linux

src/Gui/MainWindow.cpp

@@ -347,6 +347,17 @@ MainWindow::MainWindow(QWidget * parent, Qt::WindowFlags f)
 
     // accept drops on the window, get handled in dropEvent, dragEnterEvent
     setAcceptDrops(true);
+
+    // setup font substitutions for NaviCube
+    // Helvetica usually gives good enough results on mac & linux
+    // in rare cases Helvetica matches a bad font on linux
+    // Nimbus Sans Narrow and Open Sans Condensed added as fallback
+    // Bahnschrift is a condensed font available on windows versions since 2017
+    // Arial added as fallback for older version
+    auto substitutions = QStringLiteral("Bahnschrift,Helvetica,Nimbus Sans Narrow,Open Sans Condensed,Arial,Sans");
+    auto family = QStringLiteral("FreeCAD NaviCube");
+    QFont::insertSubstitutions(family, substitutions.split(QLatin1Char(',')));
+
     statusBar()->showMessage(tr("Ready"), 2001);
 }
 

src/Gui/NaviCube.cpp

@@ -325,15 +325,8 @@ void NaviCube::setBorderColor(QColor BorderColor)
 
 QString NaviCube::getDefaultSansserifFont()
 {
-    // Windows versions since 2017 have the 'Bahnschrift' font (a condensed
-    // sans serif font, optimized for readability despite being condensed),
-    // we first check for that.
-    QFont font(QStringLiteral("Bahnschrift"));
-    if (!font.exactMatch())
-        // On systems without 'Bahnschrift' we check for 'Helvetica' or its closest match
-        // as default sans serif font. (For Windows 7 this will e.g. result in 'Arial'.)
-        font = QFont(QStringLiteral("Helvetica"));
-
+    // "FreeCAD NaviCube" family susbtitutions are set in MainWindow::MainWindow
+    QFont font(QStringLiteral("FreeCAD NaviCube"));
     font.setStyleHint(QFont::SansSerif);
     // QFontInfo is required to get the actually matched font family
     return QFontInfo(font).family();