feat: Merge the tab bar underline into surrounding box borders

lib/src/components/tab_bar.dart

@@ -85,9 +85,30 @@ class TabBar extends StatelessComponent {
       ),
     );
 
-    return Row(
-      crossAxisAlignment: CrossAxisAlignment.start,
-      children: tabComponents,
+    return Stack(
+      children: [
+        // A full-width base rule painted behind the tab segments, in the
+        // border's color like the segments themselves. It fills the cells
+        // fractional pane widths leave uncovered (the segments size with
+        // maxWidth.toInt(), truncating half cells), and its ends reach one
+        // cell outside the bar to merge the underline into a surrounding
+        // box border (┝/┥). Ends with no border to join paint nothing.
+        Positioned(
+          left: 0,
+          right: 0,
+          bottom: 0,
+          child: Divider(
+            style: DividerStyle.bold,
+            indent: -1,
+            endIndent: -1,
+            color: ServerpodTheme.of(context).subtleDivider,
+          ),
+        ),
+        Row(
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: tabComponents,
+        ),
+      ],
     );
   }
 }
@@ -136,9 +157,10 @@ class _Tab extends StatelessComponent {
           ),
           Text(
             ''.padLeft(underlineWidth, '━'),
+            // The selection highlight keeps its accent color; every other
+            // underline segment matches the surrounding box border.
             style: TextStyle(
-              color: selected ? theme.activationKey : null,
-              fontWeight: selected ? FontWeight.normal : FontWeight.dim,
+              color: selected ? theme.activationKey : theme.subtleDivider,
             ),
           ),
         ],
@@ -228,7 +250,12 @@ class _TabSpacing extends StatelessComponent {
       crossAxisAlignment: CrossAxisAlignment.start,
       children: [
         Text(''.padLeft(width)),
-        Text(underline, style: const TextStyle(fontWeight: FontWeight.dim)),
+        // The underline shares the surrounding box border's color and
+        // weight so bar and border read as one frame.
+        Text(
+          underline,
+          style: TextStyle(color: ServerpodTheme.of(context).subtleDivider),
+        ),
       ],
     );
   }

test/src/components/tab_bar_test.dart

@@ -79,6 +79,119 @@ void main() {
     });
   });
 
+  test(
+    'Given a tab bar laid out at a fractional width '
+    'when rendered inside a bordered box '
+    'then the underline has no gap before the border junction',
+    () async {
+      tester = await NoctermTester.create(size: const Size(40, 6));
+      await tester.pumpComponent(
+        Container(
+          decoration: BoxDecoration(
+            border: BoxBorder.all(style: BoxBorderStyle.rounded),
+          ),
+          child: Row(
+            children: [
+              // Forces the bar onto fractional cells: its segments size
+              // with maxWidth.toInt(), which would leave the final half
+              // cell unpainted without the base rule behind them.
+              const SizedBox(width: 1.5),
+              Expanded(
+                child: Column(
+                  crossAxisAlignment: CrossAxisAlignment.stretch,
+                  children: [
+                    TabBar(
+                      labels: const ['Server logs'],
+                      selectedTab: 0,
+                      onTabChanged: (_) {},
+                    ),
+                    Expanded(child: const SizedBox.shrink()),
+                  ],
+                ),
+              ),
+            ],
+          ),
+        ),
+      );
+
+      final ts = tester.terminalState;
+      // The cell beside the border keeps the underline; a truncation gap
+      // here would break the line just before its junction.
+      expect(ts.getTextAt(38, 2, length: 1), '━');
+      expect(ts.getTextAt(39, 2, length: 1), '┥');
+    },
+  );
+
+  test(
+    'Given a tab bar inside a bordered box '
+    'when rendered then the underline merges into both border sides',
+    () async {
+      tester = await NoctermTester.create(size: const Size(40, 6));
+      await tester.pumpComponent(
+        Container(
+          decoration: BoxDecoration(
+            border: BoxBorder.all(style: BoxBorderStyle.rounded),
+          ),
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.stretch,
+            children: [
+              TabBar(
+                labels: const ['Server logs'],
+                selectedTab: 0,
+                onTabChanged: (_) {},
+              ),
+              Expanded(child: const SizedBox.shrink()),
+            ],
+          ),
+        ),
+      );
+
+      // The tab bar sits inside the border, so its label is on row 1 and
+      // its underline on row 2. The underline's ends reach the border
+      // cells and form heavy tees instead of leaving gaps.
+      final ts = tester.terminalState;
+      expect(ts.getTextAt(0, 2, length: 1), '┝');
+      expect(ts.getTextAt(39, 2, length: 1), '┥');
+      // The whole underline - junctions and segments alike - carries the
+      // surrounding box border's color, so bar and border read as one
+      // frame.
+      final borderColor = ServerpodThemeData.dark.subtleDivider;
+      expect(ts.getCellAt(0, 2)?.style.color, borderColor);
+      expect(ts.getCellAt(20, 2)?.style.color, borderColor);
+    },
+  );
+
+  test(
+    'Given a tab bar with no surrounding border '
+    'when rendered '
+    'then the underline ends paint no stray junction arms',
+    () async {
+      tester = await NoctermTester.create(size: const Size(40, 4));
+      await tester.pumpComponent(
+        Container(
+          padding: EdgeInsets.all(1),
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.stretch,
+            children: [
+              TabBar(
+                labels: const ['Server logs'],
+                selectedTab: 0,
+                onTabChanged: (_) {},
+              ),
+              Expanded(child: const SizedBox.shrink()),
+            ],
+          ),
+        ),
+      );
+
+      // The base rule reaches one cell outside the tab bar on each side,
+      // but with nothing there to join it leaves those cells untouched.
+      final ts = tester.terminalState;
+      expect(ts.getTextAt(0, 2, length: 1), ' ');
+      expect(ts.getTextAt(39, 2, length: 1), ' ');
+    },
+  );
+
   test(
     'Given fewer states than labels '
     'when rendered '