fix: enforce DataGrid cover boundary

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

@@ -94,13 +94,25 @@ assertWidths(
 const overriddenOverflowLayout = fitDataGridColumns(weightedColumns, 500, {}, { primary: 450 });
 assertWidths(
   overriddenOverflowLayout.widths,
-  { fixed: 72, primary: 328, secondary: 100 },
-  "a cover layout treats a persisted user width as preferred under viewport pressure"
+  { fixed: 72, primary: 450, secondary: 100 },
+  "a cover layout preserves an explicit user width under viewport pressure"
 );
 assertEqual(
   overriddenOverflowLayout.overflowWidth,
+  122,
+  "intentional user width may remain horizontally scrollable"
+);
+
+const overriddenUnderflowLayout = fitDataGridColumns(
+  weightedColumns,
+  800,
+  {},
+  { fixed: 80, primary: 200, secondary: 200 }
+);
+assertEqual(
+  overriddenUnderflowLayout.underflowWidth,
   0,
-  "a persisted user preference does not create avoidable horizontal overflow"
+  "a persisted cover layout is expanded when it would otherwise underflow"
 );
 
 const clampedOverrideLayout = fitDataGridColumns(weightedColumns, 800, {}, { primary: 900 }, "free");
@@ -176,15 +188,22 @@ const resizeBase = { first: 200, middle: 300, last: 300, actions: 100 };
 const resizedFirst = resizeDataGridColumn(resizeColumns, resizeBase, "first", 120, "cover");
 assertWidths(
   resizedFirst.widths,
-  { first: 320, middle: 240, last: 240, actions: 100 },
-  "right-side peers compensate an earlier cover-column resize"
+  { first: 320, middle: 300, last: 300, actions: 100 },
+  "growing an earlier cover column may create horizontal overflow"
+);
+
+const shrunkFirst = resizeDataGridColumn(resizeColumns, resizeBase, "first", -80, "cover");
+assertWidths(
+  shrunkFirst.widths,
+  { first: 120, middle: 340, last: 340, actions: 100 },
+  "shrinking an earlier cover column grows resizable columns to its right"
 );
 
 const resizedLast = resizeDataGridColumn(resizeColumns, resizeBase, "last", 120, "cover");
 assertWidths(
   resizedLast.widths,
-  { first: 140, middle: 240, last: 420, actions: 100 },
-  "left-side peers compensate the last resizable cover column"
+  { first: 200, middle: 300, last: 420, actions: 100 },
+  "the last resizable cover column may grow into horizontal overflow"
 );
 
 const committedLayout = fitDataGridColumns(
@@ -197,7 +216,72 @@ const committedLayout = fitDataGridColumns(
 assertWidths(
   committedLayout.widths,
   resizedLast.widths,
-  "committing a cover resize preserves the drag layout without a mouse-up snap"
+  "committing a cover resize preserves intentional overflow without a mouse-up snap"
+);
+
+const stoppedLastShrink = resizeDataGridColumn(
+  resizeColumns,
+  resizeBase,
+  "last",
+  -120,
+  "cover"
+);
+assertWidths(
+  stoppedLastShrink.widths,
+  resizeBase,
+  "the last resizable column stops before its shrink would require a filler"
+);
+assertEqual(
+  stoppedLastShrink.appliedDelta,
+  0,
+  "the stopped last-column shrink reports no applied movement"
+);
+
+const overflowedResizeBase = { first: 200, middle: 300, last: 420, actions: 100 };
+const consumedLastOverflow = resizeDataGridColumn(
+  resizeColumns,
+  overflowedResizeBase,
+  "last",
+  -180,
+  "cover",
+  120
+);
+assertWidths(
+  consumedLastOverflow.widths,
+  { first: 200, middle: 300, last: 300, actions: 100 },
+  "the last resizable column consumes overflow and stops at exact coverage"
+);
+assertEqual(
+  consumedLastOverflow.appliedDelta,
+  -120,
+  "last-column shrink is clamped to the available overflow"
+);
+
+const stillPastLastStop = resizeDataGridColumn(
+  resizeColumns,
+  overflowedResizeBase,
+  "last",
+  -150,
+  "cover",
+  120
+);
+assertWidths(
+  stillPastLastStop.widths,
+  consumedLastOverflow.widths,
+  "reversing the pointer while it remains beyond the stop does not move the column"
+);
+const crossedLastStop = resizeDataGridColumn(
+  resizeColumns,
+  overflowedResizeBase,
+  "last",
+  -110,
+  "cover",
+  120
+);
+assertEqual(
+  crossedLastStop.widths.last,
+  310,
+  "the column resumes only after the pointer crosses the cover boundary"
 );
 
 const fixedPeerResize = resizeDataGridColumn([
@@ -207,8 +291,8 @@ const fixedPeerResize = resizeDataGridColumn([
 ], { editable: 200, fixed: 300, actions: 100 }, "editable", 80, "cover");
 assertWidths(
   fixedPeerResize.widths,
-  { editable: 280, fixed: 220, actions: 100 },
-  "non-resizable tracks may compensate layout without exposing a resize handle"
+  { editable: 280, fixed: 300, actions: 100 },
+  "growth does not resize a fixed peer merely to avoid overflow"
 );
 
 const coverBeyondPassiveMax = resizeDataGridColumn([
@@ -218,8 +302,8 @@ const coverBeyondPassiveMax = resizeDataGridColumn([
 ], { first: 200, second: 300, actions: 100 }, "first", -80, "cover");
 assertWidths(
   coverBeyondPassiveMax.widths,
-  { first: 120, second: 380, actions: 100 },
-  "cover compensation may exceed a peer's preferred maximum to retain full coverage"
+  { first: 200, second: 300, actions: 100 },
+  "cover shrinking stops when right-side peers have reached their maximum"
 );
 
 const constrainedAtPassiveMax = resizeDataGridColumn([