fix(grid-list): tile being pulled outside the grid if no gap can be found (#9128)

Fixes and issue in the grid list where a tile, for which we couldn't find a gap in which to put it, is being pulled outside the grid due to the index defaulting to -1.

Fixes #4515.

src/lib/grid-list/grid-list.spec.ts

@@ -28,6 +28,7 @@ describe('MatGridList', () => {
         GridListWithComplexLayout,
         GridListWithFootersWithoutLines,
         GridListWithFooterContainingTwoLines,
+        GridListWithoutMatchingGap,
       ],
     });
 
@@ -287,6 +288,15 @@ describe('MatGridList', () => {
     expect(getStyle(tile, 'height')).toBe('400px');
   });
 
+  it('should ensure that all tiles are inside the grid when there are no matching gaps', () => {
+    const fixture = TestBed.createComponent(GridListWithoutMatchingGap);
+    const tiles = fixture.debugElement.queryAll(By.css('mat-grid-tile'));
+
+    fixture.detectChanges();
+    expect(tiles.every(tile => getComputedLeft(tile) >= 0))
+        .toBe(true, 'Expected none of the tiles to have a negative `left`');
+  });
+
 });
 
 
@@ -298,7 +308,7 @@ function getStyle(el: DebugElement, prop: string): string {
 function getComputedLeft(element: DebugElement): number {
   // While the other properties in this test use `getComputedStyle`, we use `getBoundingClientRect`
   // for left because iOS Safari doesn't support using `getComputedStyle` to get the calculated
-  // `left` balue when using CSS `calc`. We subtract the `left` of the document body because
+  // `left` value when using CSS `calc`. We subtract the `left` of the document body because
   // browsers, by default, add a margin to the body (typically 8px).
   let elementRect = element.nativeElement.getBoundingClientRect();
   let bodyRect = document.body.getBoundingClientRect();
@@ -458,3 +468,13 @@ class GridListWithFootersWithoutLines { }
       </mat-grid-tile>
     </mat-grid-list>`})
 class GridListWithFooterContainingTwoLines { }
+
+@Component({template: `
+  <mat-grid-list cols="5">
+    <mat-grid-tile [rowspan]="1" [colspan]="3">1</mat-grid-tile>
+    <mat-grid-tile [rowspan]="2" [colspan]="2">2</mat-grid-tile>
+    <mat-grid-tile [rowspan]="1" [colspan]="2">3</mat-grid-tile>
+    <mat-grid-tile [rowspan]="2" [colspan]="2">4</mat-grid-tile>
+  </mat-grid-list>
+`})
+class GridListWithoutMatchingGap { }

src/lib/grid-list/tile-coordinator.ts

@@ -110,7 +110,10 @@ export class TileCoordinator {
 
       // Continue iterating until we find a gap wide enough for this tile.
     } while (gapEndIndex - gapStartIndex < tileCols);
-    return gapStartIndex;
+
+    // If we still didn't manage to find a gap, ensure that the index is
+    // at least zero so the tile doesn't get pulled out of the grid.
+    return Math.max(gapStartIndex, 0);
   }
 
   /** Move "down" to the next row. */