fix: offscreen rendering with correct screen info. (#48730)

fix: osr use correct screen info.

docs/api/structures/web-preferences.md

@@ -94,6 +94,7 @@
     The actual output pixel format and color space of the texture should refer to [`OffscreenSharedTexture`](../structures/offscreen-shared-texture.md) object in the `paint` event.
     * `argb` - The requested output texture format is 8-bit unorm RGBA, with SRGB SDR color space.
     * `rgbaf16` - The requested output texture format is 16-bit float RGBA, with scRGB HDR color space.
+  * `deviceScaleFactor` number (optional) _Experimental_ - The device scale factor of the offscreen rendering output. If not set, will use primary display's scale factor as default.
 * `contextIsolation` boolean (optional) - Whether to run Electron APIs and
   the specified `preload` script in a separate JavaScript context. Defaults
   to `true`. The context that the `preload` script runs in will only have

docs/breaking-changes.md

@@ -12,6 +12,16 @@ This document uses the following convention to categorize breaking changes:
 * **Deprecated:** An API was marked as deprecated. The API will continue to function, but will emit a deprecation warning, and will be removed in a future release.
 * **Removed:** An API or feature was removed, and is no longer supported by Electron.
 
+## Planned Breaking API Changes (42.0)
+
+### Behavior Changed: Offscreen rendering will use `1.0` as default device scale factor.
+
+Previously, OSR used the primary display's device scale factor for rendering, which made the output frame size vary across users.
+Developers had to manually calculate the correct size using `screen.getPrimaryDisplay().scaleFactor`. We now provide an optional property
+`webPreferences.offscreen.deviceScaleFactor` to specify a custom value when creating an OSR window. At first, if the property is not set, it defaults
+to the primary display's scale factor (preserving the old behavior). Starting from Electron 42, the default will change to a constant value of `1.0`
+for more consistent output sizes.
+
 ## Planned Breaking API Changes (41.0)
 
 ### Behavior Changed: PDFs no longer create a separate WebContents

patches/chromium/.patches

@@ -144,3 +144,4 @@ fix_check_for_file_existence_before_setting_mtime.patch
 fix_linux_tray_id.patch
 expose_gtk_ui_platform_field.patch
 fix_os_crypt_async_cookie_encryption.patch
+patch_osr_control_screen_info.patch

patches/chromium/patch_osr_control_screen_info.patch

@@ -0,0 +1,22 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: reito <reito@chromium.org>
+Date: Wed, 29 Oct 2025 00:50:03 +0800
+Subject: patch: osr control screen info
+
+We need to override GetNewScreenInfosForUpdate to ensure the screen info
+is updated correctly, instead of overriding GetScreenInfo which seems not
+working.
+
+diff --git a/content/browser/renderer_host/render_widget_host_view_base.h b/content/browser/renderer_host/render_widget_host_view_base.h
+index c05fac0b5fc2166daaa94d407dd9a652e6144c80..9fa69b342d5efe5b2f2b39b522c543cc62c7175b 100644
+--- a/content/browser/renderer_host/render_widget_host_view_base.h
++++ b/content/browser/renderer_host/render_widget_host_view_base.h
+@@ -678,7 +678,7 @@ class CONTENT_EXPORT RenderWidgetHostViewBase
+ 
+   // Generates the most current set of ScreenInfos from the current set of
+   // displays in the system for use in UpdateScreenInfo.
+-  display::ScreenInfos GetNewScreenInfosForUpdate();
++  virtual display::ScreenInfos GetNewScreenInfosForUpdate();
+ 
+   // Called when display properties that need to be synchronized with the
+   // renderer process changes. This method is called before notifying

shell/browser/api/electron_api_web_contents.cc

@@ -826,6 +826,8 @@ WebContents::WebContents(v8::Isolate* isolate,
                              &offscreen_use_shared_texture_);
       use_offscreen_dict.Get(options::kSharedTexturePixelFormat,
                              &offscreen_shared_texture_pixel_format_);
+      use_offscreen_dict.Get(options::kDeviceScaleFactor,
+                             &offscreen_device_scale_factor_);
     }
   }
 
@@ -864,6 +866,7 @@ WebContents::WebContents(v8::Isolate* isolate,
       auto* view = new OffScreenWebContentsView(
           false, offscreen_use_shared_texture_,
           offscreen_shared_texture_pixel_format_,
+          offscreen_device_scale_factor_,
           base::BindRepeating(&WebContents::OnPaint, base::Unretained(this)));
       params.view = view;
       params.delegate_view = view;
@@ -885,7 +888,7 @@ WebContents::WebContents(v8::Isolate* isolate,
     content::WebContents::CreateParams params(session->browser_context());
     auto* view = new OffScreenWebContentsView(
         transparent, offscreen_use_shared_texture_,
-        offscreen_shared_texture_pixel_format_,
+        offscreen_shared_texture_pixel_format_, offscreen_device_scale_factor_,
         base::BindRepeating(&WebContents::OnPaint, base::Unretained(this)));
     params.view = view;
     params.delegate_view = view;
@@ -1249,6 +1252,7 @@ void WebContents::MaybeOverrideCreateParamsForNewWindow(
       auto* view = new OffScreenWebContentsView(
           false, offscreen_use_shared_texture_,
           offscreen_shared_texture_pixel_format_,
+          offscreen_device_scale_factor_,
           base::BindRepeating(&WebContents::OnPaint, base::Unretained(this)));
       create_params->view = view;
       create_params->delegate_view = view;

shell/browser/api/electron_api_web_contents.h

@@ -820,6 +820,13 @@ class WebContents final : public ExclusiveAccessContext,
   bool offscreen_use_shared_texture_ = false;
   std::string offscreen_shared_texture_pixel_format_ = "argb";
 
+  // TODO(reito): 0.0f means the device scale factor is not set, it's a
+  // migration of the breaking change so that we can read the device scale
+  // factor from physical primary screen's info. In Electron 42, we need to set
+  // this to 1.0f so that the offscreen rendering use 1.0 as default when
+  // `deviceScaleFactor` is not specified in webPreferences.
+  float offscreen_device_scale_factor_ = 0.0f;
+
   // Whether window is fullscreened by HTML5 api.
   bool html_fullscreen_ = false;
 

shell/browser/osr/osr_render_widget_host_view.cc

@@ -53,8 +53,6 @@ namespace electron {
 
 namespace {
 
-const float kDefaultScaleFactor = 1.0;
-
 ui::MouseEvent UiMouseEventFromWebMouseEvent(blink::WebMouseEvent event) {
   int button_flags = 0;
   switch (event.button) {
@@ -96,6 +94,15 @@ ui::MouseWheelEvent UiMouseWheelEventFromWebMouseEvent(
           base::ClampFloor<int>(event.delta_y)};
 }
 
+// TODO(reito): Remove this function and use default 1.0f when Electron 42.
+float GetDefaultDeviceScaleFactorFromDisplayInfo() {
+  display::Display display =
+      display::Screen::Get()->GetDisplayNearestView(gfx::NativeView());
+
+  const float factor = display.device_scale_factor();
+  return factor > 0 ? factor : 1.0f;
+}
+
 }  // namespace
 
 class ElectronDelegatedFrameHostClient
@@ -155,6 +162,7 @@ OffScreenRenderWidgetHostView::OffScreenRenderWidgetHostView(
     bool transparent,
     bool offscreen_use_shared_texture,
     const std::string& offscreen_shared_texture_pixel_format,
+    float offscreen_device_scale_factor,
     bool painting,
     int frame_rate,
     const OnPaintCallback& callback,
@@ -168,6 +176,7 @@ OffScreenRenderWidgetHostView::OffScreenRenderWidgetHostView(
       offscreen_use_shared_texture_(offscreen_use_shared_texture),
       offscreen_shared_texture_pixel_format_(
           offscreen_shared_texture_pixel_format),
+      offscreen_device_scale_factor_(offscreen_device_scale_factor),
       callback_(callback),
       frame_rate_(frame_rate),
       size_(initial_size),
@@ -184,11 +193,11 @@ OffScreenRenderWidgetHostView::OffScreenRenderWidgetHostView(
   DCHECK(render_widget_host_);
   DCHECK(!render_widget_host_->GetView());
 
-  // Initialize a screen_infos_ struct as needed, to cache the scale factor.
-  if (screen_infos_.screen_infos.empty()) {
-    UpdateScreenInfo();
+  // TODO(reito): Remove this when Electron 42.
+  if (cc::MathUtil::IsWithinEpsilon(offscreen_device_scale_factor_, 0.0f)) {
+    offscreen_device_scale_factor_ =
+        GetDefaultDeviceScaleFactorFromDisplayInfo();
   }
-  screen_infos_.mutable_current().device_scale_factor = kDefaultScaleFactor;
 
   delegated_frame_host_allocator_.GenerateId();
   delegated_frame_host_surface_id_ =
@@ -210,15 +219,6 @@ OffScreenRenderWidgetHostView::OffScreenRenderWidgetHostView(
   compositor_->SetDelegate(this);
   compositor_->SetRootLayer(root_layer_.get());
 
-  // For offscreen rendering with format rgbaf16, we need to set correct display
-  // color spaces to the compositor, otherwise it won't support hdr.
-  if (offscreen_use_shared_texture_ &&
-      offscreen_shared_texture_pixel_format_ == "rgbaf16") {
-    gfx::DisplayColorSpaces hdr_display_color_spaces(
-        gfx::ColorSpace::CreateSRGBLinear(), viz::SinglePlaneFormat::kRGBA_F16);
-    compositor_->SetDisplayColorSpaces(hdr_display_color_spaces);
-  }
-
   ResizeRootLayer(false);
 
   render_widget_host_->SetView(this);
@@ -504,19 +504,6 @@ void OffScreenRenderWidgetHostView::CopyFromSurface(
       src_rect, output_size, base::TimeDelta(), std::move(callback));
 }
 
-display::ScreenInfo OffScreenRenderWidgetHostView::GetScreenInfo() const {
-  display::ScreenInfo screen_info;
-  screen_info.depth = 24;
-  screen_info.depth_per_component = 8;
-  screen_info.orientation_angle = 0;
-  screen_info.device_scale_factor = GetDeviceScaleFactor();
-  screen_info.orientation_type =
-      display::mojom::ScreenOrientation::kLandscapePrimary;
-  screen_info.rect = gfx::Rect(size_);
-  screen_info.available_rect = gfx::Rect(size_);
-  return screen_info;
-}
-
 gfx::Rect OffScreenRenderWidgetHostView::GetBoundsInRootWindow() {
   return gfx::Rect(size_);
 }
@@ -562,8 +549,8 @@ OffScreenRenderWidgetHostView::CreateViewForWidget(
 
   return new OffScreenRenderWidgetHostView(
       transparent_, offscreen_use_shared_texture_,
-      offscreen_shared_texture_pixel_format_, true,
-      embedder_host_view->frame_rate(), callback_, render_widget_host,
+      offscreen_shared_texture_pixel_format_, offscreen_device_scale_factor_,
+      true, embedder_host_view->frame_rate(), callback_, render_widget_host,
       embedder_host_view, size());
 }
 
@@ -971,35 +958,55 @@ void OffScreenRenderWidgetHostView::InvalidateBounds(const gfx::Rect& bounds) {
   CompositeFrame(bounds);
 }
 
+display::ScreenInfos
+OffScreenRenderWidgetHostView::GetNewScreenInfosForUpdate() {
+  display::ScreenInfo screen_info;
+  screen_info.depth = 24;
+  screen_info.depth_per_component = 8;
+  screen_info.orientation_angle = 0;
+  screen_info.orientation_type =
+      display::mojom::ScreenOrientation::kLandscapePrimary;
+  screen_info.rect = gfx::Rect(size_);
+  screen_info.available_rect = gfx::Rect(size_);
+  screen_info.device_scale_factor = offscreen_device_scale_factor_;
+
+  // When pixel format is 'rgbaf16', we need to set screen info to support HDR.
+  if (offscreen_use_shared_texture_ &&
+      offscreen_shared_texture_pixel_format_ == "rgbaf16") {
+    gfx::DisplayColorSpaces hdr_display_color_spaces{
+        gfx::ColorSpace::CreateSRGBLinear(), viz::SinglePlaneFormat::kRGBA_F16};
+    // The max luminance value doesn't matter so we set to a large value.
+    hdr_display_color_spaces.SetHDRMaxLuminanceRelative(100.0f);
+    screen_info.display_color_spaces = hdr_display_color_spaces;
+  }
+
+  display::ScreenInfos screen_infos{screen_info};
+  return screen_infos;
+}
+
 void OffScreenRenderWidgetHostView::ResizeRootLayer(bool force) {
   SetupFrameRate(false);
 
-  display::Display display =
-      display::Screen::Get()->GetDisplayNearestView(GetNativeView());
-  const float scaleFactor = display.device_scale_factor();
-  float sf = GetDeviceScaleFactor();
-  const bool sf_did_change = scaleFactor != sf;
-
-  // Initialize a screen_infos_ struct as needed, to cache the scale factor.
-  if (screen_infos_.screen_infos.empty()) {
-    UpdateScreenInfo();
-  }
-  screen_infos_.mutable_current().device_scale_factor = scaleFactor;
+  auto old_screen_info = screen_infos_.current();
+  UpdateScreenInfo();
 
+  auto new_screen_info = screen_infos_.current();
   gfx::Size size = GetViewBounds().size();
 
-  if (!force && !sf_did_change && size == root_layer()->bounds().size())
+  if (!force && size == root_layer()->bounds().size() &&
+      old_screen_info == new_screen_info)
     return;
 
   root_layer()->SetBounds(gfx::Rect(size));
 
-  const gfx::Size& size_in_pixels =
-      gfx::ToFlooredSize(gfx::ConvertSizeToPixels(size, sf));
+  auto sf = GetDeviceScaleFactor();
+  const gfx::Size& size_in_pixels = SizeInPixels();
 
   if (compositor_) {
     compositor_allocator_.GenerateId();
     compositor_surface_id_ = compositor_allocator_.GetCurrentLocalSurfaceId();
     compositor_->SetScaleAndSize(sf, size_in_pixels, compositor_surface_id_);
+    compositor_->SetDisplayColorSpaces(new_screen_info.display_color_spaces);
   }
 
   delegated_frame_host_allocator_.GenerateId();

shell/browser/osr/osr_render_widget_host_view.h

@@ -73,6 +73,7 @@ class OffScreenRenderWidgetHostView
       bool transparent,
       bool offscreen_use_shared_texture,
       const std::string& offscreen_shared_texture_pixel_format,
+      float offscreen_device_scale_factor,
       bool painting,
       int frame_rate,
       const OnPaintCallback& callback,
@@ -151,7 +152,6 @@ class OffScreenRenderWidgetHostView
       base::TimeDelta timeout,
       base::OnceCallback<void(const content::CopyFromSurfaceResult&)> callback)
       override;
-  display::ScreenInfo GetScreenInfo() const override;
   void TransformPointToRootSurface(gfx::PointF* point) override {}
   gfx::Rect GetBoundsInRootWindow() override;
   std::optional<content::DisplayFeature> GetDisplayFeature() override;
@@ -171,6 +171,7 @@ class OffScreenRenderWidgetHostView
       const std::optional<std::vector<gfx::Rect>>& character_bounds) override {}
   gfx::Size GetCompositorViewportPixelSize() override;
   ui::Compositor* GetCompositor() override;
+  display::ScreenInfos GetNewScreenInfosForUpdate() override;
 
   content::RenderWidgetHostViewBase* CreateViewForWidget(
       content::RenderWidgetHost*,
@@ -293,6 +294,8 @@ class OffScreenRenderWidgetHostView
   const bool transparent_;
   const bool offscreen_use_shared_texture_;
   const std::string offscreen_shared_texture_pixel_format_;
+  float offscreen_device_scale_factor_;
+
   OnPaintCallback callback_;
   OnPopupPaintCallback parent_callback_;
 

shell/browser/osr/osr_web_contents_view.cc

@@ -17,11 +17,13 @@ OffScreenWebContentsView::OffScreenWebContentsView(
     bool transparent,
     bool offscreen_use_shared_texture,
     const std::string& offscreen_shared_texture_pixel_format,
+    float offscreen_device_scale_factor,
     const OnPaintCallback& callback)
     : transparent_(transparent),
       offscreen_use_shared_texture_(offscreen_use_shared_texture),
       offscreen_shared_texture_pixel_format_(
           offscreen_shared_texture_pixel_format),
+      offscreen_device_scale_factor_(offscreen_device_scale_factor),
       callback_(callback) {
 #if BUILDFLAG(IS_MAC)
   PlatformCreate();
@@ -116,8 +118,9 @@ OffScreenWebContentsView::CreateViewForWidget(
 
   return new OffScreenRenderWidgetHostView(
       transparent_, offscreen_use_shared_texture_,
-      offscreen_shared_texture_pixel_format_, painting_, GetFrameRate(),
-      callback_, render_widget_host, nullptr, GetSize());
+      offscreen_shared_texture_pixel_format_, offscreen_device_scale_factor_,
+      painting_, GetFrameRate(), callback_, render_widget_host, nullptr,
+      GetSize());
 }
 
 content::RenderWidgetHostViewBase*
@@ -137,9 +140,9 @@ OffScreenWebContentsView::CreateViewForChildWidget(
 
   return new OffScreenRenderWidgetHostView(
       transparent_, offscreen_use_shared_texture_,
-      offscreen_shared_texture_pixel_format_, painting_,
-      embedder_host_view->frame_rate(), callback_, render_widget_host,
-      embedder_host_view, GetSize());
+      offscreen_shared_texture_pixel_format_, offscreen_device_scale_factor_,
+      painting_, embedder_host_view->frame_rate(), callback_,
+      render_widget_host, embedder_host_view, GetSize());
 }
 
 void OffScreenWebContentsView::RenderViewReady() {

shell/browser/osr/osr_web_contents_view.h

@@ -38,6 +38,7 @@ class OffScreenWebContentsView : public content::WebContentsView,
       bool transparent,
       bool offscreen_use_shared_texture,
       const std::string& offscreen_shared_texture_pixel_format,
+      float offscreen_device_scale_factor,
       const OnPaintCallback& callback);
   ~OffScreenWebContentsView() override;
 
@@ -113,6 +114,7 @@ class OffScreenWebContentsView : public content::WebContentsView,
   const bool transparent_;
   const bool offscreen_use_shared_texture_;
   const std::string offscreen_shared_texture_pixel_format_;
+  const float offscreen_device_scale_factor_;
   bool painting_ = true;
   int frame_rate_ = 60;
   OnPaintCallback callback_;

shell/common/options_switches.h

@@ -186,6 +186,8 @@ inline constexpr std::string_view kUseSharedTexture = "useSharedTexture";
 inline constexpr std::string_view kSharedTexturePixelFormat =
     "sharedTexturePixelFormat";
 
+inline constexpr std::string_view kDeviceScaleFactor = "deviceScaleFactor";
+
 inline constexpr std::string_view kNodeIntegrationInSubFrames =
     "nodeIntegrationInSubFrames";
 

spec/api-browser-window-spec.ts

@@ -6722,6 +6722,7 @@ describe('BrowserWindow module', () => {
       expect(data.constructor.name).to.equal('NativeImage');
       expect(data.isEmpty()).to.be.false('data is empty');
       const size = data.getSize();
+      // TODO(reito): Use scale factor 1.0f when Electron 42.
       const { scaleFactor } = screen.getPrimaryDisplay();
       expect(size.width).to.be.closeTo(100 * scaleFactor, 2);
       expect(size.height).to.be.closeTo(100 * scaleFactor, 2);
@@ -6816,6 +6817,66 @@ describe('BrowserWindow module', () => {
     });
   });
 
+  describe('offscreen rendering with device scale factor', () => {
+    let w: BrowserWindow;
+    const scaleFactor = 1.5;
+
+    beforeEach(function () {
+      w = new BrowserWindow({
+        width: 100,
+        height: 100,
+        show: false,
+        webPreferences: {
+          backgroundThrottling: false,
+          offscreen: {
+            deviceScaleFactor: scaleFactor
+          }
+        }
+      });
+    });
+    afterEach(closeAllWindows);
+
+    it('creates offscreen window with correct size considering device scale factor', async () => {
+      const paint = once(w.webContents, 'paint') as Promise<[any, Electron.Rectangle, Electron.NativeImage]>;
+      w.loadFile(path.join(fixtures, 'api', 'offscreen-rendering.html'));
+      const [, , data] = await paint;
+      expect(data.constructor.name).to.equal('NativeImage');
+      expect(data.isEmpty()).to.be.false('data is empty');
+      const size = data.getSize();
+      expect(size.width).to.be.closeTo(100 * scaleFactor, 2);
+      expect(size.height).to.be.closeTo(100 * scaleFactor, 2);
+    });
+
+    it('has correct screen and window sizes', async () => {
+      w.loadFile(path.join(fixtures, 'api', 'offscreen-rendering.html'));
+      await once(w.webContents, 'dom-ready');
+      const sizes = await w.webContents.executeJavaScript(`
+        new Promise((resolve) => {
+          const screenSize = [screen.width, screen.height];
+          const outerSize = [window.outerWidth, window.outerHeight];
+          const dpr = window.devicePixelRatio;
+          resolve({ screenSize, outerSize, dpr });
+        });
+      `);
+      expect(sizes.screenSize).to.deep.equal([100, 100]);
+      expect(sizes.outerSize).to.deep.equal([100, 100]);
+      expect(sizes.dpr).to.be.equal(scaleFactor);
+    });
+
+    it('has correct device screen size media query result', async () => {
+      w.loadFile(path.join(fixtures, 'api', 'offscreen-rendering.html'));
+      await once(w.webContents, 'dom-ready');
+      const query = `(device-width: ${100}px)`;
+      const matches = await w.webContents.executeJavaScript(`
+        new Promise((resolve) => {
+          const mediaQuery = window.matchMedia('${query}');
+          resolve(mediaQuery.matches);
+        });
+      `);
+      expect(matches).to.be.true();
+    });
+  });
+
   describe('"transparent" option', () => {
     afterEach(closeAllWindows);