Fixed horizontal scroll

lib/widgets/tasq_adaptive_list.dart

@@ -230,7 +230,18 @@ class TasQAdaptiveList<T> extends StatelessWidget {
       onRowTap: onRowTap,
     );
 
-    final contentWidth = constraints.maxWidth * 0.8;
+    // Use progressively smaller fractions of the viewport on larger screens
+    // so that content doesn't stretch too widely, but also consume as much
+    // width as possible when the display is more modest.
+    double contentFactor;
+    if (constraints.maxWidth < 1200) {
+      contentFactor = 0.95; // not-so-wide monitors
+    } else if (constraints.maxWidth < 1800) {
+      contentFactor = 0.85; // wide monitors
+    } else {
+      contentFactor = 0.75; // ultra-wide monitors
+    }
+    final contentWidth = constraints.maxWidth * contentFactor;
     final tableWidth = math.max(
       contentWidth,
       (columns.length + (rowActions == null ? 0 : 1)) * 140.0,
@@ -240,25 +251,34 @@ class TasQAdaptiveList<T> extends StatelessWidget {
       math.max(1, items.length),
     );
 
-    final tableWidget = SingleChildScrollView(
-      scrollDirection: Axis.horizontal,
-      child: SizedBox(
-        width: tableWidth,
-        child: PaginatedDataTable(
-          header: tableHeader,
-          rowsPerPage: effectiveRowsPerPage,
-          columnSpacing: 20,
-          horizontalMargin: 16,
-          showCheckboxColumn: false,
-          headingRowColor: WidgetStateProperty.resolveWith(
-            (states) => Theme.of(context).colorScheme.surfaceContainer,
+    // wrap horizontal scroll with a visible scrollbar on desktop. the
+    // ScrollController is shared so the scrollbar has something to observe.
+    final horizontalController = ScrollController();
+    final tableWidget = Scrollbar(
+      controller: horizontalController,
+      thumbVisibility: true,
+      trackVisibility: true,
+      child: SingleChildScrollView(
+        controller: horizontalController,
+        scrollDirection: Axis.horizontal,
+        child: SizedBox(
+          width: tableWidth,
+          child: PaginatedDataTable(
+            header: tableHeader,
+            rowsPerPage: effectiveRowsPerPage,
+            columnSpacing: 20,
+            horizontalMargin: 16,
+            showCheckboxColumn: false,
+            headingRowColor: WidgetStateProperty.resolveWith(
+              (states) => Theme.of(context).colorScheme.surfaceContainer,
+            ),
+            columns: [
+              for (final column in columns)
+                DataColumn(label: Text(column.header)),
+              if (rowActions != null) const DataColumn(label: Text('Actions')),
+            ],
+            source: dataSource,
           ),
-          columns: [
-            for (final column in columns)
-              DataColumn(label: Text(column.header)),
-            if (rowActions != null) const DataColumn(label: Text('Actions')),
-          ],
-          source: dataSource,
         ),
       ),
     );
@@ -277,6 +297,7 @@ class TasQAdaptiveList<T> extends StatelessWidget {
       primary: true,
       child: Center(
         child: SizedBox(
+          key: const Key('adaptive_list_content'),
           width: contentWidth,
           child: Column(
             mainAxisSize: MainAxisSize.min,