Bug 1753836 - Fix callers of screenForRect to pass desktop pixels. r=jfkthame,Gijs

Device pixels and desktop pixels are not the same on macOS and Win7.
Expose the desktop-to-device scale to JS and use it appropriately.

Depends on D138038

Differential Revision: https://phabricator.services.mozilla.com/D138039

browser/base/content/tabbrowser-tabs.js

@@ -870,14 +870,11 @@
 
       // screen.availLeft et. al. only check the screen that this window is on,
       // but we want to look at the screen the tab is being dropped onto.
+      let cssToDesktopScale =
+        window.devicePixelRatio / window.desktopToDeviceScale;
       var screen = Cc["@mozilla.org/gfx/screenmanager;1"]
         .getService(Ci.nsIScreenManager)
-        .screenForRect(
-          eX * window.devicePixelRatio,
-          eY * window.devicePixelRatio,
-          1,
-          1
-        );
+        .screenForRect(eX * cssToDesktopScale, eY * cssToDesktopScale, 1, 1);
       var fullX = {},
         fullY = {},
         fullWidth = {},

browser/components/customizableui/PanelMultiView.jsm

@@ -1170,13 +1170,16 @@ var PanelMultiView = class extends AssociatedToNode {
 
     // Screen manager uses screen coordinates, while screenX/Y and anchorRect
     // are in CSS pixels, so need to convert to the right coordinate space.
-    let devicePixelRatio = this.window.devicePixelRatio;
+    let cssToDesktopPixels =
+      this.window.devicePixelRatio / this.window.desktopToDeviceScale;
     let screen = this._screenManager.screenForRect(
-      anchor.screenX * devicePixelRatio,
-      anchor.screenY * devicePixelRatio,
-      anchorRect.width * devicePixelRatio,
-      anchorRect.height * devicePixelRatio
+      anchor.screenX * cssToDesktopPixels,
+      anchor.screenY * cssToDesktopPixels,
+      anchorRect.width * cssToDesktopPixels,
+      anchorRect.height * cssToDesktopPixels
     );
+    // GetAvailRect returns screen-device pixels, which we can convert to CSS
+    // pixels here.
     let availTop = {},
       availHeight = {};
     screen.GetAvailRect({}, availTop, {}, availHeight);

browser/components/sessionstore/SessionStore.jsm

@@ -4876,13 +4876,13 @@ var SessionStoreInternal = {
   /**
    * Restore a window's dimensions
    * @param aWidth
-   *        Window width
+   *        Window width in desktop pixels
    * @param aHeight
-   *        Window height
+   *        Window height in desktop pixels
    * @param aLeft
-   *        Window left
+   *        Window left in desktop pixels
    * @param aTop
-   *        Window top
+   *        Window top in desktop pixels
    * @param aSizeMode
    *        Window size mode (eg: maximized)
    * @param aSizeModeBeforeMinimized

dom/base/nsGlobalWindowInner.cpp

@@ -211,6 +211,7 @@
 #include "nsDOMNavigationTiming.h"
 #include "nsDOMOfflineResourceList.h"
 #include "nsDebug.h"
+#include "nsDeviceContext.h"
 #include "nsDocShell.h"
 #include "nsFocusManager.h"
 #include "nsFrameMessageManager.h"
@@ -3516,28 +3517,31 @@ float nsGlobalWindowInner::GetMozInnerScreenY(CallerType aCallerType,
   FORWARD_TO_OUTER_OR_THROW(GetMozInnerScreenYOuter, (aCallerType), aError, 0);
 }
 
+static nsPresContext* GetPresContextForRatio(Document* aDoc) {
+  if (nsPresContext* presContext = aDoc->GetPresContext()) {
+    return presContext;
+  }
+  // We're in an undisplayed subdocument... There's not really an awesome way
+  // to tell what the right DPI is from here, so we try to walk up our parent
+  // document chain to the extent that the docs can observe each other.
+  Document* doc = aDoc;
+  while (doc->StyleOrLayoutObservablyDependsOnParentDocumentLayout()) {
+    doc = doc->GetInProcessParentDocument();
+    if (nsPresContext* presContext = doc->GetPresContext()) {
+      return presContext;
+    }
+  }
+  return nullptr;
+}
+
 double nsGlobalWindowInner::GetDevicePixelRatio(CallerType aCallerType,
                                                 ErrorResult& aError) {
   ENSURE_ACTIVE_DOCUMENT(aError, 0.0);
 
-  RefPtr<nsPresContext> presContext = mDoc->GetPresContext();
+  RefPtr<nsPresContext> presContext = GetPresContextForRatio(mDoc);
   if (NS_WARN_IF(!presContext)) {
-    // We're in an undisplayed subdocument... There's not really an awesome way
-    // to tell what the right DPI is from here, so we try to walk up our parent
-    // document chain to the extent that the docs can observe each other.
-    Document* doc = mDoc;
-    while (doc->StyleOrLayoutObservablyDependsOnParentDocumentLayout()) {
-      doc = doc->GetInProcessParentDocument();
-      presContext = doc->GetPresContext();
-      if (presContext) {
-        break;
-      }
-    }
-
-    if (!presContext) {
-      // Still nothing, oh well.
-      return 1.0;
-    }
+    // Still nothing, oh well.
+    return 1.0;
   }
 
   if (nsContentUtils::ResistFingerprinting(aCallerType)) {
@@ -3564,6 +3568,15 @@ double nsGlobalWindowInner::GetDevicePixelRatio(CallerType aCallerType,
          double(presContext->AppUnitsPerDevPixel());
 }
 
+double nsGlobalWindowInner::GetDesktopToDeviceScale(ErrorResult& aError) {
+  ENSURE_ACTIVE_DOCUMENT(aError, 0.0);
+  nsPresContext* presContext = GetPresContextForRatio(mDoc);
+  if (!presContext) {
+    return 1.0;
+  }
+  return presContext->DeviceContext()->GetDesktopToDeviceScale().scale;
+}
+
 uint64_t nsGlobalWindowInner::GetMozPaintCount(ErrorResult& aError) {
   FORWARD_TO_OUTER_OR_THROW(GetMozPaintCountOuter, (), aError, 0);
 }

dom/base/nsGlobalWindowInner.h

@@ -869,6 +869,7 @@ class nsGlobalWindowInner final : public mozilla::dom::EventTarget,
                            mozilla::ErrorResult& aError);
   double GetDevicePixelRatio(mozilla::dom::CallerType aCallerType,
                              mozilla::ErrorResult& aError);
+  double GetDesktopToDeviceScale(mozilla::ErrorResult& aError);
   int32_t GetScrollMinX(mozilla::ErrorResult& aError);
   int32_t GetScrollMinY(mozilla::ErrorResult& aError);
   int32_t GetScrollMaxX(mozilla::ErrorResult& aError);

dom/webidl/Window.webidl

@@ -448,6 +448,11 @@ partial interface Window {
   [Replaceable, Throws, NeedsCallerType]
   readonly attribute double devicePixelRatio;
 
+  // Allows chrome code to convert desktop pixels to device pixels and vice
+  // versa. Useful for interacting with the screen manager.
+  [ChromeOnly, Throws]
+  readonly attribute double desktopToDeviceScale;
+
   /* The maximum offset that the window can be scrolled to
      (i.e., the document width/height minus the scrollport width/height) */
   [ChromeOnly, Throws]  readonly attribute long   scrollMinX;