fix: stabilize board rails

apps/nextjs/src/app/[locale]/boards/(content)/_client.module.css

@@ -9,14 +9,46 @@
 }
 
 .gutter {
-  position: relative;
-  top: 0;
+  --board-gutter-sticky-top: calc((var(--app-shell-header-offset, 0px) + 1rem) * var(--board-canvas-inverse-scale, 1));
+  --board-gutter-viewport-height: calc(
+    (100dvh - var(--app-shell-header-offset, 0px) - 1rem) * var(--board-canvas-inverse-scale, 1)
+  );
+
+  position: sticky;
+  top: var(--board-gutter-sticky-top);
   align-self: start;
+  box-sizing: border-box;
+  height: var(--board-gutter-viewport-height);
   padding-top: 24px;
   margin-top: -24px;
-  max-height: calc(100dvh - var(--app-shell-header-offset, 0px) - var(--mantine-spacing-md));
   overflow-y: auto;
   overscroll-behavior: contain;
   scrollbar-gutter: stable;
-  transform: translateY(var(--board-gutter-sticky-offset, 0px));
+}
+
+.gutter[data-board-editing="true"] {
+  outline: 1px dashed var(--mantine-color-default-border);
+  outline-offset: -1px;
+  border-radius: var(--mantine-radius-md);
+}
+
+.gutter[data-board-editing="true"]::before {
+  position: sticky;
+  top: 0;
+  z-index: 2;
+  display: block;
+  width: fit-content;
+  height: 24px;
+  padding-inline: var(--mantine-spacing-xs);
+  margin-top: -24px;
+  overflow: hidden;
+  font-size: var(--mantine-font-size-xs);
+  line-height: 24px;
+  color: var(--mantine-color-dimmed);
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  pointer-events: none;
+  content: attr(aria-label);
+  background: color-mix(in srgb, var(--mantine-color-body) 88%, transparent);
+  border-radius: var(--mantine-radius-sm);
 }

e2e/board-grid.spec.ts

@@ -245,6 +245,14 @@ describe("Board grid", () => {
       await expect(page.locator(`[data-grid-runtime="${gridRuntimeMarker}"]`)).toHaveCount(3);
       await expect(page.getByTestId("board-grid-editor-loading")).toHaveCount(0);
       await expect(page.getByTestId("board-canvas-row-count-button")).toHaveCount(0);
+      await expect(rail).toHaveAttribute("data-board-editing", "true");
+      await expect(rail).toHaveCSS("position", "sticky");
+      await expect(rail).toHaveCSS("transform", "none");
+      const railBox = await expectBoundingBoxAsync(rail);
+      expect(railBox.height + railBox.y).toBeCloseTo(page.viewportSize()?.height ?? 0, 0);
+      expect(await rail.evaluate((element) => getComputedStyle(element, "::before").content.replaceAll('"', ""))).toBe(
+        "Left dashboard rail",
+      );
       await expect(logicalTile).toHaveCSS("overflow-x", "hidden");
       await expect(logicalTile).toHaveCSS("overflow-y", "auto");
       await logicalTile.evaluate((element) => {