fix(ui_oauth): sign in button layout

CONTRIBUTING.md

@@ -136,7 +136,7 @@ These tests require the Firebase Emulator.
 To start the Firebase Emulator, run this command:
 
 ```bash
-firebase emulator:start
+melos emulator:start
 ```
 
 Once the Firebase Emulator is up and running, execute the following Melos command from the terminal:

packages/firebase_ui_oauth/lib/src/oauth_provider_button_base.dart

@@ -388,17 +388,11 @@ class _ButtonContent extends StatelessWidget {
     required this.iconBackgroundColor,
   });
 
-  Widget _buildLoadingIndicator() {
-    return SizedBox(
-      height: fontSize,
-      width: fontSize,
-      child: loadingIndicator,
-    );
-  }
+  static const kEndPadding = 16.0;
 
   @override
   Widget build(BuildContext context) {
-    Widget child = Padding(
+    final icon = Padding(
       padding: EdgeInsets.all(iconPadding),
       child: SvgPicture.string(
         iconSrc,
@@ -407,39 +401,96 @@ class _ButtonContent extends StatelessWidget {
       ),
     );
 
-    if (label.isNotEmpty) {
-      final content = isLoading
-          ? _buildLoadingIndicator()
-          : Text(
-              label,
-              textAlign: TextAlign.center,
-              style: TextStyle(
-                height: 1.1,
-                color: textColor,
-                fontSize: fontSize,
-              ),
-            );
-
-      final isCupertino = CupertinoUserInterfaceLevel.maybeOf(context) != null;
-      final topMargin = isCupertino ? (height - fontSize) / 2 : 0.0;
+    if (label.isEmpty) {
+      if (!isLoading) return icon;
 
-      child = Stack(
-        children: [
-          child,
-          Align(
-            alignment: AlignmentDirectional.center,
-            child: Padding(
-              padding: EdgeInsets.only(top: topMargin),
-              child: content,
-            ),
-          ),
-        ],
+      return SizedBox.square(
+        dimension: fontSize,
+        child: loadingIndicator,
       );
-    } else if (isLoading) {
-      child = _buildLoadingIndicator();
     }
 
-    return child;
+    final text = TextSpan(
+      text: label,
+      style: TextStyle(
+        height: 1.1,
+        color: textColor,
+        fontSize: fontSize,
+      ),
+    );
+
+    return Row(
+      children: [
+        //
+        icon,
+        //
+        Expanded(
+          child: LayoutBuilder(
+            builder: (context, constraints) {
+              //
+              final availableWidth = constraints.maxWidth;
+
+              // "height" is also the button's width
+              final totalIconWidth = height + (iconPadding * 2);
+              final totalButtonWidth =
+                  availableWidth + totalIconWidth + kEndPadding;
+
+              if (isLoading) {
+                // "fontSize" is also the indicator's width
+                final freeSpace = totalButtonWidth - fontSize;
+                final startPadding =
+                    (freeSpace / 2 - totalIconWidth).clamp(0.0, availableWidth);
+
+                return Padding(
+                  padding: EdgeInsetsDirectional.only(
+                    start: startPadding,
+                  ),
+                  child: Align(
+                    alignment: AlignmentDirectional.centerStart,
+                    child: SizedBox.square(
+                      dimension: fontSize,
+                      child: loadingIndicator,
+                    ),
+                  ),
+                );
+              }
+
+              final isCupertino =
+                  CupertinoUserInterfaceLevel.maybeOf(context) != null;
+              final topPadding = isCupertino ? (height - fontSize) / 2 : 0.0;
+
+              final textWidth = (TextPainter(
+                text: text,
+                textDirection: Directionality.of(context),
+                maxLines: 1,
+              )..layout(maxWidth: availableWidth))
+                  .size
+                  .width;
+              final freeSpace = totalButtonWidth - textWidth;
+              final startPadding =
+                  (freeSpace / 2 - totalIconWidth).clamp(0.0, availableWidth);
+
+              return Padding(
+                padding: EdgeInsetsDirectional.only(
+                  top: topPadding,
+                  start: startPadding,
+                ),
+                child: FittedBox(
+                  fit: BoxFit.scaleDown,
+                  alignment: AlignmentDirectional.centerStart,
+                  child: Text.rich(
+                    text,
+                    style: text.style,
+                  ),
+                ),
+              );
+            },
+          ),
+        ),
+        //
+        const SizedBox(width: kEndPadding),
+      ],
+    );
   }
 }
 

packages/firebase_ui_oauth/test/flutterfire_ui_oauth_test.dart

@@ -205,11 +205,13 @@ void main() {
           home: Scaffold(
             body: Row(
               children: [
-                OAuthProviderButtonBase(
-                  provider: provider,
-                  auth: FakeAuth(),
-                  label: 'Sign in with Fake provider',
-                  loadingIndicator: const CircularProgressIndicator(),
+                Expanded(
+                  child: OAuthProviderButtonBase(
+                    provider: provider,
+                    auth: FakeAuth(),
+                    label: 'Sign in with Fake provider',
+                    loadingIndicator: const CircularProgressIndicator(),
+                  ),
                 )
               ],
             ),