Fix responsive DataGrid contraction

webui/tests/data-grid-sizing.test.ts

@@ -219,6 +219,52 @@ assertWidths(
   "committing a cover resize preserves intentional overflow without a mouse-up snap"
 );
 
+const contractedUserLayout = fitDataGridColumns(
+  resizeColumns,
+  700,
+  {},
+  resizedLast.widths,
+  "cover",
+  900
+);
+assertEqual(
+  contractedUserLayout.overflowWidth,
+  120,
+  "container contraction retains only the overflow deliberately selected by the user"
+);
+assertEqual(
+  Object.values(contractedUserLayout.widths).reduce((total, width) => total + width, 0),
+  820,
+  "persisted tracks contract with their container instead of retaining the first pixel layout"
+);
+assertEqual(
+  Object.entries(contractedUserLayout.widths).every(([columnId, width]) => {
+    const column = resizeColumns.find((candidate) => candidate.id === columnId);
+    return Boolean(column && width >= effectiveDataGridColumnMinWidth(column));
+  }),
+  true,
+  "responsive user layouts retain every hard column minimum"
+);
+
+const contractedCoveredLayout = fitDataGridColumns(
+  resizeColumns,
+  700,
+  {},
+  resizeBase,
+  "cover",
+  900
+);
+assertEqual(
+  contractedCoveredLayout.overflowWidth,
+  0,
+  "an exact-cover user layout continues to cover a narrower container without avoidable overflow"
+);
+assertEqual(
+  Object.values(contractedCoveredLayout.widths).reduce((total, width) => total + width, 0),
+  700,
+  "an exact-cover layout shrinks to the newly available width"
+);
+
 const stoppedLastShrink = resizeDataGridColumn(
   resizeColumns,
   resizeBase,
@@ -361,3 +407,8 @@ assertEqual(
   false,
   "free and cover layouts keep independent user overrides"
 );
+assertEqual(
+  originalSignature.startsWith("v2::"),
+  true,
+  "the responsive persistence contract invalidates legacy hard-width snapshots"
+);