Make default wxSizer border DPI-aware.

Scale the (still hard-coded) border in pixels by the content scale factor for
the platforms where this needs to be done, i.e. not wxGTK nor wxOSX where the
underlying toolkit already does it.

docs/changes.txt

@@ -123,6 +123,7 @@ wxGTK:
 
 wxMSW:
 
+- Make default wxSizer border DPI-aware.
 - Improve wxMimeTypesManager open command detection (Eric Jensen).
 - Make wxFILTER_INCLUDE_LIST in wxTextValidator actually usable.
 - Fix appearance of toggled wxToggleButtons with bitmap (tm).

include/wx/sizer.h

@@ -117,10 +117,21 @@ public:
         // GNOME HIG says to use 6px as the base unit:
         // http://library.gnome.org/devel/hig-book/stable/design-window.html.en
         return 6;
-    #else
-        // FIXME: default border size shouldn't be hardcoded and at the very
-        //        least they should depend on the current font size
+    #elif defined(__WXMAC__)
+        // Not sure if this is really the correct size for the border.
         return 5;
+    #else
+        // For the other platforms, we need to scale raw pixel values using the
+        // current DPI, do it once (and cache the result) in another function.
+        #define wxNEEDS_BORDER_IN_PX
+
+        // We don't react to dynamic DPI changes, so we can cache the values of
+        // the border in on-screen pixels after computing it once. This
+        // could/should change in the future.
+        if ( !ms_defaultBorderInPx )
+            ms_defaultBorderInPx = DoGetDefaultBorderInPx();
+
+        return ms_defaultBorderInPx;
     #endif
 #else
         return 0;
@@ -222,6 +233,12 @@ public:
     int GetBorderInPixels() const { return m_borderInPixels; }
 
 private:
+#ifdef wxNEEDS_BORDER_IN_PX
+    static int DoGetDefaultBorderInPx();
+
+    static int ms_defaultBorderInPx;
+#endif // wxNEEDS_BORDER_IN_PX
+
     int m_proportion;
     int m_flags;
     int m_borderInPixels;

interface/wx/sizer.h

@@ -1372,6 +1372,10 @@ public:
     /**
         Sets the wxSizerFlags to have a border of a number of pixels specified
         by @a borderinpixels with the directions specified by @a direction.
+
+        Prefer to use the overload below or DoubleBorder() or TripleBorder()
+        versions instead of hard-coding the border value in pixels to avoid too
+        small borders on devices with high DPI displays.
     */
     wxSizerFlags& Border(int direction, int borderinpixels);
 
@@ -1478,6 +1482,10 @@ public:
 
     /**
         Returns the border used by default in Border() method.
+
+        This value is scaled appropriately for the current DPI on the systems
+        where physical pixel values are used for the control positions and
+        sizes, i.e. not with wxGTK or wxOSX.
     */
     static int GetDefaultBorder();
 

src/common/sizer.cpp

@@ -84,6 +84,34 @@ WX_DEFINE_EXPORTED_LIST( wxSizerItemList )
     minsize
 */
 
+// ----------------------------------------------------------------------------
+// wxSizerFlags
+// ----------------------------------------------------------------------------
+
+#ifdef wxNEEDS_BORDER_IN_PX
+
+int wxSizerFlags::ms_defaultBorderInPx = 0;
+
+/* static */
+int wxSizerFlags::DoGetDefaultBorderInPx()
+{
+    // Hard code 5px as it's the minimal border size between two controls, see
+    // the table at the bottom of
+    // https://msdn.microsoft.com/en-us/library/windows/desktop/dn742486.aspx
+    //
+    // Of course, ideal would be to use the appropriate sizes for the borders
+    // between related and unrelated controls, as explained at the above URL,
+    // but we don't have a way to specify this in our API currently.
+    //
+    // We also have to use the DPI for the primary monitor here as we don't
+    // have any associated window, so this is wrong on systems using multiple
+    // monitors with different resolutions too -- but, again, without changes
+    // in the API, there is nothing we can do about this.
+    return wxWindow::FromDIP(5, NULL);
+}
+
+#endif // wxNEEDS_BORDER_IN_PX
+
 // ----------------------------------------------------------------------------
 // wxSizerItem
 // ----------------------------------------------------------------------------