Make TabBar's tab label padding configurable (#21758)

packages/flutter/lib/src/material/tabs.dart

@@ -534,6 +534,7 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
     this.indicatorSize,
     this.labelColor,
     this.labelStyle,
+    this.labelPadding,
     this.unselectedLabelColor,
     this.unselectedLabelStyle,
   }) : assert(tabs != null),
@@ -637,6 +638,11 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
   /// definition is used.
   final TextStyle labelStyle;
 
+  /// The padding added to each of the tab labels.
+  ///
+  /// If this property is null then kTabLabelPadding is used.
+  final EdgeInsetsGeometry labelPadding;
+
   /// The text style of the unselected tab labels
   ///
   /// If this property is null then the [labelStyle] value is used. If [labelStyle]
@@ -891,7 +897,7 @@ class _TabBarState extends State<TabBar> {
       wrappedTabs[i] = Center(
         heightFactor: 1.0,
         child: Padding(
-          padding: kTabLabelPadding,
+          padding: widget.labelPadding ?? kTabLabelPadding,
           child: KeyedSubtree(
             key: _tabKeys[i],
             child: widget.tabs[i],

packages/flutter/test/material/tabs_test.dart

@@ -1291,6 +1291,76 @@ void main() {
     ));
   });
 
+  testWidgets('TabBar with labelPadding', (WidgetTester tester) async {
+    const double indicatorWeight = 2.0; // default indicator weight
+    const EdgeInsets labelPadding = EdgeInsets.only(left: 3.0, right: 7.0);
+    const EdgeInsets indicatorPadding = labelPadding;
+
+    final List<Widget> tabs = <Widget>[
+      SizedBox(key: UniqueKey(), width: 130.0, height: 30.0),
+      SizedBox(key: UniqueKey(), width: 140.0, height: 40.0),
+      SizedBox(key: UniqueKey(), width: 150.0, height: 50.0),
+    ];
+
+    final TabController controller = TabController(
+      vsync: const TestVSync(),
+      length: tabs.length,
+    );
+
+    await tester.pumpWidget(
+      boilerplate(
+        child: Container(
+          alignment: Alignment.topLeft,
+          child: TabBar(
+            labelPadding: labelPadding,
+            indicatorPadding: labelPadding,
+            isScrollable: true,
+            controller: controller,
+            tabs: tabs,
+          ),
+        ),
+      ),
+    );
+
+    final RenderBox tabBarBox = tester.firstRenderObject<RenderBox>(find.byType(TabBar));
+    const double tabBarHeight = 50.0 + indicatorWeight;  // 50 = max tab height
+    expect(tabBarBox.size.height, tabBarHeight);
+
+    // Tab0 width = 130, height = 30
+    double tabLeft = labelPadding.left;
+    double tabRight = tabLeft + 130.0;
+    double tabTop = (tabBarHeight - indicatorWeight - 30.0) / 2.0;
+    double tabBottom = tabTop + 30.0;
+    Rect tabRect = Rect.fromLTRB(tabLeft, tabTop, tabRight, tabBottom);
+    expect(tester.getRect(find.byKey(tabs[0].key)), tabRect);
+
+    // Tab1 width = 140, height = 40
+    tabLeft = tabRight + labelPadding.right + labelPadding.left;
+    tabRight = tabLeft + 140.0;
+    tabTop = (tabBarHeight - indicatorWeight - 40.0) / 2.0;
+    tabBottom = tabTop + 40.0;
+    tabRect = Rect.fromLTRB(tabLeft, tabTop, tabRight, tabBottom);
+    expect(tester.getRect(find.byKey(tabs[1].key)), tabRect);
+
+    // Tab2 width = 150, height = 50
+    tabLeft = tabRight + labelPadding.right + labelPadding.left;
+    tabRight = tabLeft + 150.0;
+    tabTop = (tabBarHeight - indicatorWeight - 50.0) / 2.0;
+    tabBottom = tabTop + 50.0;
+    tabRect = Rect.fromLTRB(tabLeft, tabTop, tabRight, tabBottom);
+    expect(tester.getRect(find.byKey(tabs[2].key)), tabRect);
+
+    // Tab 0 selected, indicatorPadding == labelPadding
+    final double indicatorLeft = indicatorPadding.left + indicatorWeight / 2.0;
+    final double indicatorRight = 130.0 + labelPadding.horizontal - indicatorPadding.right - indicatorWeight / 2.0;
+    final double indicatorY = tabBottom + indicatorWeight / 2.0;
+    expect(tabBarBox, paints..line(
+      strokeWidth: indicatorWeight,
+      p1: Offset(indicatorLeft, indicatorY),
+      p2: Offset(indicatorRight, indicatorY),
+    ));
+  });
+
   testWidgets('Overflowing RTL tab bar', (WidgetTester tester) async {
     final List<Widget> tabs = List<Widget>.filled(100,
       // For convenience padded width of each tab will equal 100: