src/QmlControls/ScreenTools.qml

 pragma Singleton
 
-import QtQuick 2.3
+import QtQuick          2.12
 import QtQuick.Controls 1.2
-import QtQuick.Window 2.2
+import QtQuick.Window   2.2
 
 import QGroundControl                       1.0
 import QGroundControl.ScreenToolsController 1.0
@@ -43,6 +43,9 @@ Item {
     /// your ui elements will reposition themselves appropriately on varying screen sizes and resolutions.
     property real defaultFontPixelWidth:    10
 
+    /// QFontMetrics::descent for default font at default point size
+    property real defaultFontDescent:       0
+
     property real smallFontPointSize:       10
     property real mediumFontPointSize:      10
     property real largeFontPointSize:       10
@@ -134,6 +137,7 @@ Item {
         defaultFontPointSize    = pointSize
         defaultFontPixelHeight  = Math.round(_textMeasure.fontHeight/2.0)*2
         defaultFontPixelWidth   = Math.round(_textMeasure.fontWidth/2.0)*2
+        defaultFontDescent      = ScreenToolsController.defaultFontDescent(defaultFontPointSize)
         smallFontPointSize      = defaultFontPointSize  * _screenTools.smallFontPointRatio
         mediumFontPointSize     = defaultFontPointSize  * _screenTools.mediumFontPointRatio
         largeFontPointSize      = defaultFontPointSize  * _screenTools.largeFontPointRatio

src/QmlControls/ScreenToolsController.cc

@@ -73,3 +73,8 @@ ScreenToolsController::boldFontFamily() const
         return QString("opensans-demibold");
     }
 }
+
+double ScreenToolsController::defaultFontDescent(int pointSize) const
+{
+    return QFontMetrics(QFont(normalFontFamily(), pointSize)).descent();
+}

src/QmlControls/ScreenToolsController.h

@@ -47,6 +47,9 @@ public:
     Q_INVOKABLE int mouseX(void) { return QCursor::pos().x(); }
     Q_INVOKABLE int mouseY(void) { return QCursor::pos().y(); }
 
+    // QFontMetrics::descent for default font
+    Q_INVOKABLE double defaultFontDescent(int pointSize) const;
+
 #if defined(__mobile__)
     bool    isMobile            () const { return true;  }
 #else
@@ -103,12 +106,11 @@ public:
     bool isDebug                () { return false; }
 #endif
 
-    bool    hasTouch() const;
-
-    QString  iOSDevice          () const;
-    QString  fixedFontFamily    () const;
-    QString  normalFontFamily   () const;
-    QString  boldFontFamily     () const;
+    bool        hasTouch            () const;
+    QString     iOSDevice           () const;
+    QString     fixedFontFamily     () const;
+    QString     normalFontFamily    () const;
+    QString     boldFontFamily      () const;
 };
 
 #endif