fix: enforce DataGrid cover boundary

webui/src/components/table/DataGrid.tsx

@@ -135,9 +135,11 @@ type DataGridBaseProps<T> = {
   /** @deprecated Use initialFit instead. */
   fit?: DataGridInitialFit;
   /**
-   * Cover keeps the right edge fixed by redistributing every resize across
-   * real columns. Free permits underflow and horizontal growth. Free is
-   * automatically promoted to cover when a right-sticky column exists.
+   * Cover prevents underflow while allowing horizontal overflow. Shrinking
+   * consumes existing overflow and then grows eligible columns to the right;
+   * it stops when no such capacity remains. Free permits both underflow and
+   * overflow. Free is automatically promoted to cover when a right-sticky
+   * column exists.
    */
   resizeBehavior?: DataGridResizeBehavior;
   className?: string;

webui/src/components/table/dataGridSizing.ts

@@ -210,15 +210,8 @@ export function fitDataGridColumns(
         resizeTargetForLayout(column, widths, widths[column.id])
       )
     );
-    // A persisted user layout is preferred, not a reason to overflow. Under
-    // viewport pressure every ordinary track may shrink down to its hard floor.
-    remaining = applyDataGridDistribution(
-      widths,
-      remaining,
-      coverageColumns.map((column) =>
-        resizeTargetForLayout(column, widths, widths[column.id])
-      )
-    );
+    // Explicit user widths are hard during reconciliation. Cover prevents
+    // underflow, but intentional user growth may remain horizontally scrollable.
   }
 
   const roundedWidths = roundDataGridWidths(widths);
@@ -268,6 +261,43 @@ export function resizeDataGridColumn(
     };
   }
 
+  if (fitMode === "cover") {
+    if (boundedDelta >= 0) {
+      widths[columnId] = startWidth + boundedDelta;
+      return {
+        widths: roundDataGridWidths(widths),
+        appliedDelta: roundWidth(boundedDelta)
+      };
+    }
+
+    const requestedShrink = -boundedDelta;
+    const freeShrink = Math.min(
+      requestedShrink,
+      Math.max(0, uncompensatedShrinkRoom)
+    );
+    const rightTargets = columns
+      .slice(activeIndex + 1)
+      .filter((column) => column.resizable && !column.sticky)
+      .map((column) =>
+        resizeTargetForLayout(
+          column,
+          widths,
+          dataGridColumnResizeWeight(column)
+        )
+      );
+    const compensation = distributeDataGridResizeAmount(
+      requestedShrink - freeShrink,
+      rightTargets
+    );
+    applyDataGridAmounts(widths, compensation.amounts);
+    const appliedShrink = freeShrink + compensation.applied;
+    widths[columnId] = startWidth - appliedShrink;
+    return {
+      widths: roundDataGridWidths(widths),
+      appliedDelta: roundWidth(-appliedShrink)
+    };
+  }
+
   const freeShrink = boundedDelta < 0
     ? Math.min(-boundedDelta, Math.max(0, uncompensatedShrinkRoom))
     : 0;
@@ -282,8 +312,7 @@ export function resizeDataGridColumn(
       resizeTargetForLayout(
         column,
         widths,
-        dataGridColumnResizeWeight(column),
-        fitMode === "cover" ? DATA_GRID_MAX_TRACK_WIDTH : undefined
+        dataGridColumnResizeWeight(column)
       )
     );
     const distribution = distributeDataGridResizeAmount(remainingCompensation, targets);