bug 794038 pt 0.9 - provide an unscaledDevicePixelsPerCSSPixel attribute on nsIBaseWindow to expose the default device-pix/css-pix ratio. r=roc

docshell/base/nsDocShell.cpp

@@ -4933,6 +4933,23 @@ nsDocShell::Destroy()
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDocShell::GetUnscaledDevicePixelsPerCSSPixel(double *aScale)
+{
+    if (mParentWidget) {
+        *aScale = mParentWidget->GetDefaultScale();
+        return NS_OK;
+    }
+
+    nsCOMPtr<nsIBaseWindow> ownerWindow(do_QueryInterface(mTreeOwner));
+    if (ownerWindow) {
+        return ownerWindow->GetUnscaledDevicePixelsPerCSSPixel(aScale);
+    }
+
+    *aScale = 1.0;
+    return NS_OK;
+}
+
 NS_IMETHODIMP
 nsDocShell::SetPosition(int32_t x, int32_t y)
 {

embedding/browser/webBrowser/nsDocShellTreeOwner.cpp

@@ -470,6 +470,17 @@ nsDocShellTreeOwner::Destroy()
   return NS_ERROR_NULL_POINTER;
 }
 
+NS_IMETHODIMP
+nsDocShellTreeOwner::GetUnscaledDevicePixelsPerCSSPixel(double *aScale)
+{
+  if (mWebBrowser) {
+    return mWebBrowser->GetUnscaledDevicePixelsPerCSSPixel(aScale);
+  }
+
+  *aScale = 1.0;
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsDocShellTreeOwner::SetPosition(int32_t aX, int32_t aY)
 {

embedding/browser/webBrowser/nsWebBrowser.cpp

@@ -1239,6 +1239,12 @@ NS_IMETHODIMP nsWebBrowser::Destroy()
    return NS_OK;
 }
 
+NS_IMETHODIMP nsWebBrowser::GetUnscaledDevicePixelsPerCSSPixel(double *aScale)
+{
+  *aScale = mParentWidget ? mParentWidget->GetDefaultScale() : 1.0;
+  return NS_OK;
+}
+
 NS_IMETHODIMP nsWebBrowser::SetPosition(int32_t aX, int32_t aY)
 {
    int32_t cx = 0;

widget/nsIBaseWindow.idl

@@ -21,7 +21,7 @@ typedef voidPtr nativeWindow;
  * but rather a common set that nearly all windowed objects support.    
  */
 
-[scriptable, uuid(7144AC8B-6702-4A4B-A73D-D1D4E9717E46)]
+[scriptable, uuid(9DA319F3-EEE6-4504-81A5-6A19CF6215BF)]
 interface nsIBaseWindow : nsISupports
 {
 	/*
@@ -182,7 +182,16 @@ interface nsIBaseWindow : nsISupports
 	lives if it has not had to create its own widget.
 	*/
 	[noscript] readonly attribute nsIWidget mainWidget;
-	
+
+	/*
+	The number of	device pixels per CSS pixel used on this window's current
+	screen at the default zoom level.
+	This is the value returned by GetDefaultScale() of the underlying widget.
+	Note that this may change if the window is moved between screens with
+	differing resolutions.
+	*/
+	readonly attribute double unscaledDevicePixelsPerCSSPixel;
+
 	/**
 	* Give the window focus.
 	*/

xpfe/appshell/src/nsChromeTreeOwner.cpp

@@ -348,6 +348,12 @@ NS_IMETHODIMP nsChromeTreeOwner::Destroy()
    return mXULWindow->Destroy();
 }
 
+NS_IMETHODIMP nsChromeTreeOwner::GetUnscaledDevicePixelsPerCSSPixel(double *aScale)
+{
+   NS_ENSURE_STATE(mXULWindow);
+   return mXULWindow->GetUnscaledDevicePixelsPerCSSPixel(aScale);
+}
+
 NS_IMETHODIMP nsChromeTreeOwner::SetPosition(int32_t x, int32_t y)
 {
    NS_ENSURE_STATE(mXULWindow);

xpfe/appshell/src/nsContentTreeOwner.cpp

@@ -565,6 +565,12 @@ NS_IMETHODIMP nsContentTreeOwner::Destroy()
    return mXULWindow->Destroy();
 }
 
+NS_IMETHODIMP nsContentTreeOwner::GetUnscaledDevicePixelsPerCSSPixel(double* aScale)
+{
+   NS_ENSURE_STATE(mXULWindow);
+   return mXULWindow->GetUnscaledDevicePixelsPerCSSPixel(aScale);
+}
+
 NS_IMETHODIMP nsContentTreeOwner::SetPosition(int32_t aX, int32_t aY)
 {
    NS_ENSURE_STATE(mXULWindow);

xpfe/appshell/src/nsXULWindow.cpp

@@ -531,6 +531,12 @@ NS_IMETHODIMP nsXULWindow::Destroy()
   return NS_OK;
 }
 
+NS_IMETHODIMP nsXULWindow::GetUnscaledDevicePixelsPerCSSPixel(double *aScale)
+{
+  *aScale = mWindow ? mWindow->GetDefaultScale() : 1.0;
+  return NS_OK;
+}
+
 NS_IMETHODIMP nsXULWindow::SetPosition(int32_t aX, int32_t aY)
 {
   // Don't reset the window's size mode here - platforms that don't want to move