virtual-scroll: add tests for fixed size items (#11255)

* virtual-scroll: add tests for fixed size items

* address comments

* fix bug in fixed size strategy

* fix bazel build

Author: mmalerba

src/cdk-experimental/scrolling/BUILD.bazel

@@ -10,6 +10,7 @@ ng_module(
   module_name = "@angular/cdk-experimental/scrolling",
   assets = [":virtual_scroll_viewport_css"] + glob(["**/*.html"]),
   deps = [
+    "//src/cdk/coercion",
     "//src/cdk/collections",
     "@rxjs",
   ],

src/cdk-experimental/scrolling/fixed-size-virtual-scroll.ts

@@ -94,10 +94,12 @@ export class FixedSizeVirtualScrollStrategy implements VirtualScrollStrategy {
 
     const scrollOffset = this._viewport.measureScrollOffset();
     const firstVisibleIndex = Math.floor(scrollOffset / this._itemSize);
+    const firstItemRemainder = scrollOffset % this._itemSize;
     const range = this._expandRange(
         {start: firstVisibleIndex, end: firstVisibleIndex},
         this._bufferSize,
-        Math.ceil(this._viewport.getViewportSize() / this._itemSize) + this._bufferSize);
+        Math.ceil((this._viewport.getViewportSize() + firstItemRemainder) / this._itemSize) +
+            this._bufferSize);
     this._viewport.setRenderedRange(range);
     this._viewport.setRenderedContentOffset(this._itemSize * range.start);
   }