Scale window content with screen DPI on Windows

os/win/window.cpp

@@ -158,6 +158,7 @@ WindowWin::Touch::Touch() : fingers(0), canBeMouse(false), asMouse(false), timer
 WindowWin::WindowWin(const WindowSpec& spec)
   : m_clientSize(1, 1)
   , m_scale(spec.scale())
+  , m_baseScale(spec.scale())
   , m_isCreated(false)
   , m_adjustShadow(true)
   , m_translateDeadKeys(false)
@@ -263,6 +264,9 @@ WindowWin::WindowWin(const WindowSpec& spec)
 
   SetWindowLongPtr(m_hwnd, GWLP_USERDATA, reinterpret_cast<LONG_PTR>(this));
 
+  if (winApi.GetWindowDpiAwarenessContext)
+    m_scale = MulDiv(m_baseScale, winApi.GetDpiForWindow(m_hwnd), USER_DEFAULT_SCREEN_DPI);
+
   // This flag is used to avoid calling T::resizeImpl() when we
   // add the scrollbars to the window. (As the T type could not be
   // fully initialized yet.)
@@ -361,7 +365,9 @@ os::ColorSpaceRef WindowWin::colorSpace() const
 
 void WindowWin::setScale(int scale)
 {
-  m_scale = scale;
+  double scaleFactor = m_scale / (double) m_baseScale;
+  m_baseScale = scale;
+  m_scale = scale * scaleFactor;
 
   // Align window size to new scale
   {
@@ -966,6 +972,20 @@ LRESULT WindowWin::wndProc(UINT msg, WPARAM wparam, LPARAM lparam)
       }
       break;
 
+    case WM_DPICHANGED: {
+      m_scale = MulDiv(m_baseScale, HIWORD(wparam), USER_DEFAULT_SCREEN_DPI);
+
+      RECT* prcNewWindow = (RECT*)lparam;
+      SetWindowPos(m_hwnd,
+                  nullptr,
+                  prcNewWindow ->left,
+                  prcNewWindow ->top,
+                  prcNewWindow->right - prcNewWindow->left,
+                  prcNewWindow->bottom - prcNewWindow->top,
+                  SWP_NOZORDER | SWP_NOACTIVATE);
+      break;
+    }
+
     case WM_SETCURSOR:
       // We set our custom cursor if we are in the client area, or in
       // the case of windows with custom frames (borderless), we

os/win/window.h

@@ -39,6 +39,7 @@ class WindowWin : public Window {
   os::ScreenRef screen() const override;
   os::ColorSpaceRef colorSpace() const override;
   int scale() const override { return m_scale; }
+  int baseScale() const override { return m_baseScale; }
   void setScale(int scale) override;
   bool isVisible() const override;
   void setVisible(bool visible) override;
@@ -144,6 +145,7 @@ class WindowWin : public Window {
   std::unique_ptr<DragTargetAdapter> m_dragTargetAdapter = nullptr;
 
   int m_scale;
+  int m_baseScale;
   bool m_isCreated;
   // Since Windows Vista, it looks like Microsoft decided to change
   // the meaning of the window position to the shadow position (when

os/window.h

@@ -103,6 +103,10 @@ class Window : public RefCount {
   // The window size will be a multiple of the scale.
   virtual void setScale(int scale) = 0;
 
+  // Returns the current window scale for default DPI value.
+  // It is constant across different displays.
+  virtual int baseScale() const { return scale(); }
+
   // Returns true if the window is visible in the screen.
   virtual bool isVisible() const = 0;