fix(cdk/overlay): sub-pixel deviations throwing off positioning in some cases

If the browser is zoomed in beyond the default level, it may report numbers with
sub-pixel deviations to `getBoundingClientRect` (e.g. 100.09 vs 100) which can throw off
our logic when comparing against the viewport size which is always a whole number.

These changes fix the issue by rounding down the numbers that we get from `getBoundingClientRect`.

Fixes #21350.

Author: crisbeto

src/cdk/overlay/position/flexible-connected-position-strategy.spec.ts

@@ -1,8 +1,8 @@
 import {ComponentPortal, PortalModule} from '@angular/cdk/portal';
 import {CdkScrollable, ScrollingModule, ViewportRuler} from '@angular/cdk/scrolling';
-import {MockNgZone} from '@angular/cdk/testing/private';
+import {dispatchFakeEvent, MockNgZone} from '@angular/cdk/testing/private';
 import {Component, ElementRef, NgModule, NgZone} from '@angular/core';
-import {inject, TestBed} from '@angular/core/testing';
+import {fakeAsync, inject, TestBed, tick} from '@angular/core/testing';
 import {Subscription} from 'rxjs';
 import {map} from 'rxjs/operators';
 import {
@@ -2194,7 +2194,51 @@ describe('FlexibleConnectedPositionStrategy', () => {
         expect(Math.floor(overlayRect.width)).toBe(rightOffset);
       });
 
+    it('should account for sub-pixel deviations in the size of the overlay', fakeAsync(() => {
+      originElement.style.top = '200px';
+      originElement.style.left = '200px';
+
+      positionStrategy
+        .withFlexibleDimensions()
+        .withPositions([{
+          originX: 'start',
+          originY: 'bottom',
+          overlayX: 'start',
+          overlayY: 'top'
+        }]);
+
+      attachOverlay({
+        positionStrategy,
+        height: '100%'
+      });
 
+      const originalGetBoundingClientRect = overlayRef.overlayElement.getBoundingClientRect;
+
+      // The browser may return a `ClientRect` with sub-pixel deviations if the screen is zoomed in.
+      // Since there's no way for us to zoom in the screen programmatically, we simulate the effect
+      // by patching `getBoundingClientRect` to return a slightly different value.
+      overlayRef.overlayElement.getBoundingClientRect = function() {
+        const clientRect = originalGetBoundingClientRect.apply(this);
+        const zoomOffset = 0.1;
+
+        return {
+          top: clientRect.top,
+          right: clientRect.right + zoomOffset,
+          bottom: clientRect.bottom + zoomOffset,
+          left: clientRect.left,
+          width: clientRect.width + zoomOffset,
+          height: clientRect.height + zoomOffset
+        } as any;
+      };
+
+      // Trigger a resize so that the overlay get repositioned from scratch
+      // and to have it use the patched `getBoundingClientRect`.
+      dispatchFakeEvent(window, 'resize');
+      tick(100); // The resize listener is usually debounced.
+
+      const overlayRect = originalGetBoundingClientRect.apply(overlayRef.overlayElement);
+      expect(Math.floor(overlayRect.top)).toBe(0);
+    }));
 
   });
 

src/cdk/overlay/position/flexible-connected-position-strategy.ts

@@ -595,7 +595,7 @@ export class FlexibleConnectedPositionStrategy implements PositionStrategy {
    *     originPoint.
    */
   private _pushOverlayOnScreen(start: Point,
-                               overlay: ClientRect,
+                               rawOverlayRect: ClientRect,
                                scrollPosition: ViewportScrollPosition): Point {
     // If the position is locked and we've pushed the overlay already, reuse the previous push
     // amount, rather than pushing it again. If we were to continue pushing, the element would
@@ -607,6 +607,9 @@ export class FlexibleConnectedPositionStrategy implements PositionStrategy {
       };
     }
 
+    // Round the overlay rect when comparing against the
+    // viewport, because the viewport is always rounded.
+    const overlay = getRoundedBoundingClientRect(rawOverlayRect);
     const viewport = this._viewportRect;
 
     // Determine how much the overlay goes outside the viewport on each
@@ -1219,3 +1222,20 @@ function getPixelValue(input: number|string|null|undefined): number|null {
 
   return input || null;
 }
+
+/**
+ * Gets a version of an element's bounding `ClientRect` where all the values are rounded down to
+ * the nearest pixel. This allows us to account for the cases where there may be sub-pixel
+ * deviations in the `ClientRect` returned by the browser (e.g. when zoomed in with a percentage
+ * size, see #21350).
+ */
+function getRoundedBoundingClientRect(clientRect: ClientRect): ClientRect {
+  return {
+    top: Math.floor(clientRect.top),
+    right: Math.floor(clientRect.right),
+    bottom: Math.floor(clientRect.bottom),
+    left: Math.floor(clientRect.left),
+    width: Math.floor(clientRect.width),
+    height: Math.floor(clientRect.height)
+  };
+}