tor-browser

commit d45de9c0f73a5fe192041547d6b0f64135a62b78
parent 4e64d65b0b2e2a2dac151ffaad6f6b455c69a53c
Author: Emilio Cobos Álvarez <emilio@crisal.io>
Date:   Wed,  3 Dec 2025 07:13:12 +0000

Bug 1998555 - Make sure that nsIBaseWindow scale agrees with nsDeviceContext. r=jwatt

At some resolutions (like 1.75x) the difference with the app units
rounding that nsDeviceContext does eventually is enough to cause a few
pixels of difference for a big screen.

Make sure that nsIBaseWindow accounts for the resolution the same way
layout and rendering eventually do.

Differential Revision: https://phabricator.services.mozilla.com/D274749

Diffstat:
M
gfx/src/nsDeviceContext.cpp
 | 
31
+++++++++++++++++++------------
M
gfx/src/nsDeviceContext.h
 | 
4
++++
M
widget/nsIBaseWindow.cpp
 | 
9
+++++++--

3 files changed, 30 insertions(+), 14 deletions(-)
diff --git a/gfx/src/nsDeviceContext.cpp b/gfx/src/nsDeviceContext.cpp
@@ -44,6 +44,19 @@ nsDeviceContext::nsDeviceContext()
 
 nsDeviceContext::~nsDeviceContext() = default;
 
+int32_t nsDeviceContext::ComputeAppUnitsPerDevPixelForWidgetScale(
+    CSSToLayoutDeviceScale aScale) {
+  return std::max(1, NS_lround(AppUnitsPerCSSPixel() / aScale.scale));
+}
+
+int32_t nsDeviceContext::ApplyFullZoomToAPD(int32_t aUnzoomedAppUnits,
+                                            float aFullZoom) {
+  if (aFullZoom == 1.0f) {
+    return aUnzoomedAppUnits;
+  }
+  return std::max(1, NSToIntRound(float(aUnzoomedAppUnits) / aFullZoom));
+}
+
 void nsDeviceContext::SetDPI() {
   float dpi;
 
@@ -53,7 +66,8 @@ void nsDeviceContext::SetDPI() {
     mPrintingScale = mDeviceContextSpec->GetPrintingScale();
     mPrintingTranslate = mDeviceContextSpec->GetPrintingTranslate();
     mAppUnitsPerDevPixelAtUnitFullZoom =
-        NS_lround((AppUnitsPerCSSPixel() * 96) / dpi);
+        ComputeAppUnitsPerDevPixelForWidgetScale(
+            CSSToLayoutDeviceScale(96.0f / dpi));
   } else {
     // A value of -1 means use the maximum of 96 and the system DPI.
     // A value of 0 means use the system DPI. A positive value is used as the
@@ -76,7 +90,7 @@ void nsDeviceContext::SetDPI() {
         mWidget ? mWidget->GetDefaultScale() : CSSToLayoutDeviceScale(1.0);
     MOZ_ASSERT(scale.scale > 0.0);
     mAppUnitsPerDevPixelAtUnitFullZoom =
-        std::max(1, NS_lround(AppUnitsPerCSSPixel() / scale.scale));
+        ComputeAppUnitsPerDevPixelForWidgetScale(scale);
   }
 
   NS_ASSERTION(dpi != -1.0, "no dpi set");
@@ -365,22 +379,15 @@ bool nsDeviceContext::SetFullZoom(float aScale) {
   return oldAppUnitsPerDevPixel != mAppUnitsPerDevPixel;
 }
 
-static int32_t ApplyFullZoom(int32_t aUnzoomedAppUnits, float aFullZoom) {
-  if (aFullZoom == 1.0f) {
-    return aUnzoomedAppUnits;
-  }
-  return std::max(1, NSToIntRound(float(aUnzoomedAppUnits) / aFullZoom));
-}
-
 int32_t nsDeviceContext::AppUnitsPerDevPixelInTopLevelChromePage() const {
   // The only zoom that applies to chrome pages is the system zoom, if any.
-  return ApplyFullZoom(mAppUnitsPerDevPixelAtUnitFullZoom,
-                       LookAndFeel::SystemZoomSettings().mFullZoom);
+  return ApplyFullZoomToAPD(mAppUnitsPerDevPixelAtUnitFullZoom,
+                            LookAndFeel::SystemZoomSettings().mFullZoom);
 }
 
 void nsDeviceContext::UpdateAppUnitsForFullZoom() {
   mAppUnitsPerDevPixel =
-      ApplyFullZoom(mAppUnitsPerDevPixelAtUnitFullZoom, mFullZoom);
+      ApplyFullZoomToAPD(mAppUnitsPerDevPixelAtUnitFullZoom, mFullZoom);
   // adjust mFullZoom to reflect appunit rounding
   mFullZoom = float(mAppUnitsPerDevPixelAtUnitFullZoom) / mAppUnitsPerDevPixel;
 }
diff --git a/gfx/src/nsDeviceContext.h b/gfx/src/nsDeviceContext.h
@@ -91,6 +91,10 @@ class nsDeviceContext final {
    */
   int32_t AppUnitsPerDevPixel() const { return mAppUnitsPerDevPixel; }
 
+  static int32_t ComputeAppUnitsPerDevPixelForWidgetScale(
+      mozilla::CSSToLayoutDeviceScale);
+  static int32_t ApplyFullZoomToAPD(int32_t aAppUnitsPerPixel, float aFullZoom);
+
   /**
    * Convert device pixels which is used for gfx/thebes to nearest
    * (rounded) app units
diff --git a/widget/nsIBaseWindow.cpp b/widget/nsIBaseWindow.cpp
@@ -4,11 +4,16 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsIBaseWindow.h"
+#include "nsDeviceContext.h"
 #include "mozilla/LookAndFeel.h"
 
 using namespace mozilla;
 
 CSSToLayoutDeviceScale nsIBaseWindow::UnscaledDevicePixelsPerCSSPixel() {
-  return CSSToLayoutDeviceScale(GetWidgetCSSToDeviceScale() *
-                                LookAndFeel::SystemZoomSettings().mFullZoom);
+  CSSToLayoutDeviceScale widgetScale(GetWidgetCSSToDeviceScale());
+  int32_t apd =
+      nsDeviceContext::ComputeAppUnitsPerDevPixelForWidgetScale(widgetScale);
+  apd = nsDeviceContext::ApplyFullZoomToAPD(
+      apd, LookAndFeel::SystemZoomSettings().mFullZoom);
+  return CSSToLayoutDeviceScale(double(AppUnitsPerCSSPixel()) / double(apd));
 }