Edit by @mklim: Comment-511798199 found the Samsung issue. The first few comments in the chain refer to earlier versions of Flutter without any keyboard support at all. Quoting the relevant comment below.

Maybe Samsung mobile phones have this problem，I tried two Samsung phones.
samsung galaxy s6 edge+
samsung galaxy S7（G9300)

The problem may be that it won't get the focus for the first time , you need to click the input box again.

Please see the gif below：

flutter: Channel master, v1.8.2-pre.42
webview_flutter: ^0.3.10+2

My environment is as follows:

/Users/mayer/development/flutter/bin/flutter doctor --verbose
[✓] Flutter (Channel master, v1.8.2-pre.42, on Mac OS X 10.14.4 18E226, locale zh-Hans-CN)
    • Flutter version 1.8.2-pre.42 at /Users/mayer/development/flutter
    • Framework revision 35b07a1f7f (8 hours ago), 2019-07-15 23:43:27 -0400
    • Engine revision 74af88ba9f
    • Dart version 2.5.0 (build 2.5.0-dev.1.0 24725a8559)

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    • Android SDK at /Users/mayer/Library/Android/sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-28, build-tools 28.0.3
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 10.2.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 10.2.1, Build version 10E1001
    • CocoaPods version 1.6.1

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

[✓] Android Studio (version 3.4)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 37.0.1
    • Dart plugin version 183.6270
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)

[✓] Connected device (3 available)
    • SM G9280 • 0815f8acd00f1b04 • android-arm64  • Android 7.0 (API 24)
    • macOS    • macOS            • darwin-x64     • Mac OS X 10.14.4 18E226
    • Chrome   • chrome           • web-javascript • Google Chrome 75.0.3770.142 


• No issues found!
Process finished with exit code 0

Run the example in the webview_flutter plugin.
When I typed on the pop-up keyboard, the text I entered was not displayed.

Please see the gif below：

flutter 1.7.8
webview_flutter 0.3.9+1

My environment is as follows:

/Users/mayer/development/flutter/bin/flutter doctor --verbose
[✓] Flutter (Channel stable, v1.7.8+hotfix.3, on Mac OS X 10.14.4 18E226, locale zh-Hans-CN)
    • Flutter version 1.7.8+hotfix.3 at /Users/mayer/development/flutter
    • Framework revision b712a172f9 (15 hours ago), 2019-07-09 13:14:38 -0700
    • Engine revision 54ad777fd2
    • Dart version 2.4.0

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    • Android SDK at /Users/mayer/Library/Android/sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-28, build-tools 28.0.3
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 10.2.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 10.2.1, Build version 10E1001
    • CocoaPods version 1.6.1

[✓] iOS tools - develop for iOS devices
    • ios-deploy 1.9.4

[✓] Android Studio (version 3.4)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 35.3.1
    • Dart plugin version 183.6270
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)

[✓] Connected device (1 available)
    • SM G9280 • 0815f8acd00f1b04 • android-arm64 • Android 7.0 (API 24)

• No issues found!
Process finished with exit code 0