fix(canvas): fill background to prevent gaps when scrolling to edges

- Fill entire canvas with grid background color before rendering
- Update scrollbar CSS to use 8px instead of 15px
- Prevents empty areas when content width is less than view width

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: sealday

packages/sheet/src/sheet/hooks/useTableRender.ts

@@ -305,9 +305,17 @@ export function useTableRender(data: DataProxy | null) {
     const fh = rows.height;
 
     // 调整 canvas 大小
-    draw.resize(data.viewWidth(), data.viewHeight());
+    const viewWidth = data.viewWidth();
+    const viewHeight = data.viewHeight();
+    draw.resize(viewWidth, viewHeight);
     draw.clear();
 
+    // 先填充整个背景（防止滚动到边缘时出现空白区域）
+    draw.save();
+    draw.attr({ fillStyle: RenderConfig.grid.backgroundColor });
+    draw.fillRect(0, 0, viewWidth, viewHeight);
+    draw.restore();
+
     const viewRange = data.viewRange();
     const tx = data.freezeTotalWidth();
     const ty = data.freezeTotalHeight();

packages/sheet/src/sheet/sheet.less

@@ -303,7 +303,7 @@ body {
   opacity: 0.9;
   z-index: 12;
   &.horizontal {
-    right: 15px;
+    right: 8px;
     overflow-x: scroll;
     overflow-y: hidden;
     > div {
@@ -312,7 +312,7 @@ body {
     }
   }
   &.vertical {
-    bottom: 15px;
+    bottom: 8px;
     overflow-x: hidden;
     overflow-y: scroll;
     > div {