Steps to reproduce

On the master channel, canvas.drawLine no longer correctly renders with a Paint and Color with an alpha channel.

First run on the stable channel to see the expected (correct) behaviour:

git clone https://github.com/nmfisher/flutter_macos_drawline_alpha_regression
flutter channel stable
flutter run -d macos 

On the current stable channel (3.44.2), the left and right columns (drawLine) correctly fade in the same manner as the middle column (drawRect).

Switch to master channel, clean & re-run the project to see the regression:

flutter channel master
flutter clean
flutter run -d macos

On the current master channel (3.45.0-1.0.pre-387), the left and right columns are all rendered at full opacity. Paint()..color = Colors.white.withValues(alpha: a) and Paint()..color = Color.fromARGB(ai, 255, 255, 255) are no longer respected.

Expected results

As above

Actual results

As above

Code sample

As above

Screenshots or Video

As above

Logs

No response

Flutter Doctor output

nickfisher@Nicks-Mac-mini stroke_alpha_repro % flutter doctor -v
[✓] Flutter (Channel master, 3.45.0-1.0.pre-596, on macOS 26.4.1 25E253 darwin-arm64, locale en-SG) [4.2s]
• Flutter version 3.45.0-1.0.pre-596 on channel master at /Users/nickfisher/Downloads/flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision e228771 (7 hours ago), 2026-06-22 05:51:36 -0400
• Engine revision e228771
• Dart version 3.13.0 (build 3.13.0-229.0.dev)
• DevTools version 2.59.0
• Feature flags: enable-web, enable-linux-desktop, enable-macos-desktop, enable-windows-desktop, enable-android, enable-ios, cli-animations, enable-native-assets, enable-swift-package-manager, omit-legacy-version-file, enable-lldb-debugging,
enable-uiscene-migration, enable-riscv64

[!] Android toolchain - develop for Android devices (Android SDK version 35.0.1) [1,648ms]
• Android SDK at /Users/nickfisher/Library/Android/sdk
• Emulator version 36.1.9.0 (build_id 13823996) (CL:N/A)
• Platform android-36, build-tools 35.0.1
• Java binary at: /Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
This is the JDK bundled with the latest Android Studio installation on this machine.
To manually set the JDK path, use: flutter config --jdk-dir="path/to/jdk".
• Java version OpenJDK Runtime Environment (build 21.0.6+-13391695-b895.109)
! Some Android licenses not accepted. To resolve this, run: flutter doctor --android-licenses

[✓] Xcode - develop for iOS and macOS (Xcode 26.5) [1,056ms]
• Xcode at /Applications/Xcode.app/Contents/Developer
• Build 17F42
• CocoaPods version 1.16.2

[✓] Chrome - develop for the web [6ms]
• Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Connected device (3 available) [6.7s]
• Nick Fisher’s iPhone (wireless) (mobile) • 00008030-000465A236BA802E • ios • iOS 17.6.1 21G93
• macOS (desktop) • macos • darwin-arm64 • macOS 26.4.1 25E253 darwin-arm64
• Chrome (web) • chrome • web-javascript • Google Chrome 149.0.7827.115

[✓] Network resources [911ms]
• All expected network resources are available.

! Doctor found issues in 1 category.