Add framework for Per-Monitor DPI Awareness on Windows

React to the WM_DPICHANGED event and update the size of the  child windows and
the top-level window. Scale the minimum and maximum window size to the new DPI.

Only react to WM_DPICHANGED when DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2 is
used.

include/wx/defs.h

@@ -2830,6 +2830,7 @@ WX_MSW_DECLARE_HANDLE(HBITMAP);
 WX_MSW_DECLARE_HANDLE(HIMAGELIST);
 WX_MSW_DECLARE_HANDLE(HGLOBAL);
 WX_MSW_DECLARE_HANDLE(HDC);
+WX_MSW_DECLARE_HANDLE(DPI_AWARENESS_CONTEXT);
 typedef WXHINSTANCE WXHMODULE;
 
 #undef WX_MSW_DECLARE_HANDLE

include/wx/msw/missing.h

@@ -23,7 +23,11 @@
 #endif
 
 #ifndef WM_PRINTCLIENT
-    #define WM_PRINTCLIENT 0x318
+    #define WM_PRINTCLIENT      0x0318
+#endif
+
+#ifndef WM_DPICHANGED
+    #define WM_DPICHANGED       0x02E0
 #endif
 
 #ifndef DT_HIDEPREFIX

include/wx/msw/toplevel.h

@@ -165,6 +165,9 @@ protected:
                                           int& x, int& y,
                                           int& w, int& h) const wxOVERRIDE;
 
+    // WM_DPICHANGED handler.
+    bool HandleDPIChange(const wxSize& newDPI, const wxRect& newRect);
+
     // This field contains the show command to use when showing the window the
     // next time and also indicates whether the window should be considered
     // being iconized or maximized (which may be different from whether it's
@@ -191,6 +194,14 @@ protected:
     wxWindowRef m_winLastFocused;
 
 private:
+    // Keep track of the DPI used in this window. So when per-monitor dpi
+    // awareness is enabled, both old and new DPI are known for
+    // wxDPIChangedEvent and wxWindow::MSWUpdateOnDPIChange.
+    wxSize m_activeDPI;
+
+    // This window supports handling per-monitor DPI awareness when the
+    // application manifest contains <dpiAwareness>PerMonitorV2</dpiAwareness>.
+    bool m_perMonitorDPIaware;
 
     // The system menu: initially NULL but can be set (once) by
     // MSWGetSystemMenu(). Owned by this window.

include/wx/msw/window.h

@@ -586,6 +586,10 @@ public:
     // Should be overridden by all classes storing the "last focused" window.
     virtual void WXDoUpdatePendingFocus(wxWindow* WXUNUSED(win)) {}
 
+    // Called from WM_DPICHANGED handler for all windows to let them update
+    // any sizes and fonts used internally when the DPI changes.
+    void MSWUpdateOnDPIChange(const wxSize& oldDPI, const wxSize& newDPI);
+
 protected:
     // this allows you to implement standard control borders without
     // repeating the code in different classes that are not derived from

src/msw/toplevel.cpp

@@ -104,6 +104,9 @@ void wxTopLevelWindowMSW::Init()
     m_fsIsShowing = false;
 
     m_menuSystem = NULL;
+
+    m_activeDPI = wxDefaultSize;
+    m_perMonitorDPIaware = false;
 }
 
 WXDWORD wxTopLevelWindowMSW::MSWGetStyle(long style, WXDWORD *exflags) const
@@ -246,6 +249,26 @@ WXHWND wxTopLevelWindowMSW::MSWGetParent() const
     return (WXHWND)hwndParent;
 }
 
+bool wxTopLevelWindowMSW::HandleDPIChange(const wxSize& newDPI, const wxRect& newRect)
+{
+    if ( !m_perMonitorDPIaware )
+    {
+        return false;
+    }
+
+    if ( newDPI != m_activeDPI )
+    {
+        MSWUpdateOnDPIChange(m_activeDPI, newDPI);
+        m_activeDPI = newDPI;
+    }
+
+    SetSize(newRect);
+
+    Refresh();
+
+    return true;
+}
+
 WXLRESULT wxTopLevelWindowMSW::MSWWindowProc(WXUINT message, WXWPARAM wParam, WXLPARAM lParam)
 {
     WXLRESULT rc = 0;
@@ -306,6 +329,17 @@ WXLRESULT wxTopLevelWindowMSW::MSWWindowProc(WXUINT message, WXWPARAM wParam, WX
 #endif // #ifndef __WXUNIVERSAL__
             }
             break;
+
+        case WM_DPICHANGED:
+            {
+                const RECT* const prcNewWindow =
+                                         reinterpret_cast<const RECT*>(lParam);
+
+                processed = HandleDPIChange(wxSize(LOWORD(wParam),
+                                                   HIWORD(wParam)),
+                                            wxRectFromRECT(*prcNewWindow));
+            }
+            break;
     }
 
     if ( !processed )
@@ -314,6 +348,47 @@ WXLRESULT wxTopLevelWindowMSW::MSWWindowProc(WXUINT message, WXWPARAM wParam, WX
     return rc;
 }
 
+namespace
+{
+
+static bool IsPerMonitorDPIAware(HWND hwnd)
+{
+    bool dpiAware = false;
+
+    // Determine if 'Per Monitor v2' DPI awareness is enabled in the
+    // applications manifest.
+#if wxUSE_DYNLIB_CLASS
+    #define WXDPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2 ((WXDPI_AWARENESS_CONTEXT)-4)
+    typedef WXDPI_AWARENESS_CONTEXT(WINAPI * GetWindowDpiAwarenessContext_t)(HWND hwnd);
+    typedef BOOL(WINAPI * AreDpiAwarenessContextsEqual_t)(WXDPI_AWARENESS_CONTEXT dpiContextA, WXDPI_AWARENESS_CONTEXT dpiContextB);
+    static GetWindowDpiAwarenessContext_t s_pfnGetWindowDpiAwarenessContext = NULL;
+    static AreDpiAwarenessContextsEqual_t s_pfnAreDpiAwarenessContextsEqual = NULL;
+    static bool s_initDone = false;
+
+    if ( !s_initDone )
+    {
+        wxLoadedDLL dllUser32("user32.dll");
+        wxDL_INIT_FUNC(s_pfn, GetWindowDpiAwarenessContext, dllUser32);
+        wxDL_INIT_FUNC(s_pfn, AreDpiAwarenessContextsEqual, dllUser32);
+        s_initDone = true;
+    }
+
+    if ( s_pfnGetWindowDpiAwarenessContext && s_pfnAreDpiAwarenessContextsEqual )
+    {
+        WXDPI_AWARENESS_CONTEXT dpiAwarenessContext = s_pfnGetWindowDpiAwarenessContext(hwnd);
+
+        if ( s_pfnAreDpiAwarenessContextsEqual(dpiAwarenessContext, WXDPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2) == TRUE )
+        {
+            dpiAware = true;
+        }
+    }
+#endif // wxUSE_DYNLIB_CLASS
+
+    return dpiAware;
+}
+
+}
+
 bool wxTopLevelWindowMSW::CreateDialog(const void *dlgTemplate,
                                        const wxString& title,
                                        const wxPoint& pos,
@@ -484,6 +559,10 @@ bool wxTopLevelWindowMSW::Create(wxWindow *parent,
         EnableCloseButton(false);
     }
 
+    m_activeDPI = GetDPI();
+
+    m_perMonitorDPIaware = IsPerMonitorDPIAware(GetHwnd());
+
     // for standard dialogs the dialog manager generates WM_CHANGEUISTATE
     // itself but for custom windows we have to do it ourselves in order to
     // make the keyboard indicators (such as underlines for accelerators and

src/msw/window.cpp

@@ -4837,6 +4837,47 @@ wxSize wxWindowMSW::GetDPI() const
     return dpi;
 }
 
+// Helper function to update the given coordinate by the scaling factor if it
+// is set, i.e. different from wxDefaultCoord.
+static void ScaleCoordIfSet(int& coord, float scaleFactor)
+{
+    if ( coord != wxDefaultCoord )
+    {
+        const float coordScaled = coord * scaleFactor;
+        coord = scaleFactor > 1.0 ? ceil(coordScaled) : floor(coordScaled);
+    }
+}
+
+void
+wxWindowMSW::MSWUpdateOnDPIChange(const wxSize& oldDPI, const wxSize& newDPI)
+{
+    // update min and max size if necessary
+    const float scaleFactor = (float)newDPI.y / oldDPI.y;
+
+    ScaleCoordIfSet(m_minHeight, scaleFactor);
+    ScaleCoordIfSet(m_minWidth, scaleFactor);
+    ScaleCoordIfSet(m_maxHeight, scaleFactor);
+    ScaleCoordIfSet(m_maxWidth, scaleFactor);
+
+    InvalidateBestSize();
+
+    // update children
+    wxWindowList::compatibility_iterator current = GetChildren().GetFirst();
+    while ( current )
+    {
+        wxWindow *childWin = current->GetData();
+        // Update all children, except other top-level windows.
+        // These could be on a different monitor and will get their own
+        // dpi-changed event.
+        if ( childWin && !childWin->IsTopLevel() )
+        {
+            childWin->MSWUpdateOnDPIChange(oldDPI, newDPI);
+        }
+
+        current = current->GetNext();
+    }
+}
+
 // ---------------------------------------------------------------------------
 // colours and palettes
 // ---------------------------------------------------------------------------