feat: add transparent webpreference to webview (#40301)

* feat: add transparent option to WebContents

* feat: add transparent attribute to webview

* test: add tests for webview transparent attribute

* docs: add transparent attribute to webview docs

* fix: run tests on macOS only

* refactor: remove unneeded html tag

* fix: only apply transparent option to guests

* refactor: correct comment

* refactor: use opaque instead

Retains current webview behaviour by default.

* fix: correct variable name to guest_opaque_

* refactor: use transparent webpreference

* docs: remove unused web preference

* fix: uncomment condition for transparency test

* docs: converted to list format and linked to MDN

* fix: make webviews transparent by default again

* fix: rebase error

---------

Co-authored-by: Cheng Zhao <zcbenz@gmail.com>

docs/api/webview-tag.md

@@ -221,7 +221,9 @@ windows. Popups are disabled by default.
 ```
 
 A `string` which is a comma separated list of strings which specifies the web preferences to be set on the webview.
-The full list of supported preference strings can be found in [BrowserWindow](browser-window.md#new-browserwindowoptions).
+The full list of supported preference strings can be found in [BrowserWindow](browser-window.md#new-browserwindowoptions). In addition, webview supports the following preferences:
+
+* `transparent` boolean (optional) - Whether to enable background transparency for the guest page. Default is `true`. **Note:** The guest page's text and background colors are derived from the [color scheme](https://developer.mozilla.org/en-US/docs/Web/CSS/color-scheme) of its root element. When transparency is enabled, the text color will still change accordingly but the background will remain transparent.
 
 The string follows the same format as the features string in `window.open`.
 A name by itself is given a `true` boolean value.

shell/browser/api/electron_api_web_contents.cc

@@ -822,6 +822,9 @@ WebContents::WebContents(v8::Isolate* isolate,
   // Get type
   options.Get("type", &type_);
 
+  // Get transparent for guest view
+  options.Get("transparent", &guest_transparent_);
+
   bool b = false;
   if (options.Get(options::kOffscreen, &b) && b)
     type_ = Type::kOffScreen;
@@ -3778,7 +3781,7 @@ void WebContents::SetImageAnimationPolicy(const std::string& new_policy) {
 }
 
 void WebContents::SetBackgroundColor(absl::optional<SkColor> maybe_color) {
-  SkColor color = maybe_color.value_or(type_ == Type::kWebView ||
+  SkColor color = maybe_color.value_or((IsGuest() && guest_transparent_) ||
                                                type_ == Type::kBrowserView
                                            ? SK_ColorTRANSPARENT
                                            : SK_ColorWHITE);

shell/browser/api/electron_api_web_contents.h

@@ -801,6 +801,9 @@ class WebContents : public ExclusiveAccessContext,
   // The type of current WebContents.
   Type type_ = Type::kBrowserWindow;
 
+  // Weather the guest view should be transparent
+  bool guest_transparent_ = true;
+
   int32_t id_;
 
   // Request id used for findInPage request.

spec/fixtures/pages/flex-webview.html

@@ -0,0 +1,15 @@
+<style>
+    html,
+    body {
+        height: 100%;
+    }
+
+    body {
+        display: flex;
+        margin: 0;
+    }
+    
+    webview {
+        flex: 1;
+    }
+</style>

spec/webview-spec.ts

@@ -1,6 +1,6 @@
 import * as path from 'node:path';
 import * as url from 'node:url';
-import { BrowserWindow, session, ipcMain, app, WebContents } from 'electron/main';
+import { BrowserWindow, session, ipcMain, app, WebContents, screen } from 'electron/main';
 import { closeAllWindows } from './lib/window-helpers';
 import { emittedUntil } from './lib/events-helpers';
 import { ifit, ifdescribe, defer, itremote, useRemoteContext, listen } from './lib/spec-helpers';
@@ -9,6 +9,7 @@ import * as http from 'node:http';
 import * as auth from 'basic-auth';
 import { once } from 'node:events';
 import { setTimeout } from 'node:timers/promises';
+import { areColorsSimilar, captureScreen, HexColors, getPixelColor } from './lib/screen-helpers';
 
 declare let WebView: any;
 const features = process._linkedBinding('electron_common_features');
@@ -773,6 +774,85 @@ describe('<webview> tag', function () {
     });
   });
 
+  describe('webpreferences attribute', () => {
+    const WINDOW_BACKGROUND_COLOR = '#55ccbb';
+
+    let w: BrowserWindow;
+    before(async () => {
+      w = new BrowserWindow({
+        webPreferences: {
+          webviewTag: true,
+          nodeIntegration: true,
+          contextIsolation: false
+        }
+      });
+      await w.loadURL(`file://${fixtures}/pages/flex-webview.html`);
+      w.setBackgroundColor(WINDOW_BACKGROUND_COLOR);
+    });
+    afterEach(async () => {
+      await w.webContents.executeJavaScript(`{
+        for (const el of document.querySelectorAll('webview')) el.remove();
+      }`);
+    });
+    after(() => w.close());
+
+    // Linux and arm64 platforms (WOA and macOS) do not return any capture sources
+    ifit(process.platform === 'darwin' && process.arch === 'x64')('is transparent by default', async () => {
+      await loadWebView(w.webContents, {
+        src: 'data:text/html,foo'
+      });
+
+      await setTimeout(1000);
+
+      const display = screen.getPrimaryDisplay();
+      const screenCapture = await captureScreen();
+      const centerColor = getPixelColor(screenCapture, {
+        x: display.size.width / 2,
+        y: display.size.height / 2
+      });
+
+      expect(areColorsSimilar(centerColor, WINDOW_BACKGROUND_COLOR)).to.be.true();
+    });
+
+    // Linux and arm64 platforms (WOA and macOS) do not return any capture sources
+    ifit(process.platform === 'darwin' && process.arch === 'x64')('remains transparent when set', async () => {
+      await loadWebView(w.webContents, {
+        src: 'data:text/html,foo',
+        webpreferences: 'transparent=yes'
+      });
+
+      await setTimeout(1000);
+
+      const display = screen.getPrimaryDisplay();
+      const screenCapture = await captureScreen();
+      const centerColor = getPixelColor(screenCapture, {
+        x: display.size.width / 2,
+        y: display.size.height / 2
+      });
+
+      expect(areColorsSimilar(centerColor, WINDOW_BACKGROUND_COLOR)).to.be.true();
+    });
+
+    // Linux and arm64 platforms (WOA and macOS) do not return any capture sources
+    ifit(process.platform === 'darwin' && process.arch === 'x64')('can disable transparency', async () => {
+      await loadWebView(w.webContents, {
+        src: 'data:text/html,foo',
+        webpreferences: 'transparent=no'
+      });
+
+      await setTimeout(1000);
+
+      const display = screen.getPrimaryDisplay();
+      const screenCapture = await captureScreen();
+      const centerColor = getPixelColor(screenCapture, {
+        x: display.size.width / 2,
+        y: display.size.height / 2
+      });
+
+      expect(areColorsSimilar(centerColor, HexColors.WHITE)).to.be.true();
+    });
+  });
+
   describe('permission request handlers', () => {
     let w: BrowserWindow;
     beforeEach(async () => {