test: fix datepicker position tests failing in firefox v78

The datepicker position tests seem to be thrown off in Firefox v78 where
inputs have an uneven width (resulting in subpixel rendering that is not
predictable and stable enough for our test assertions)

Author: devversion

src/material/datepicker/datepicker.spec.ts

@@ -1929,6 +1929,20 @@ describe('MatDatepicker', () => {
 
 });
 
+/**
+ * Styles that set input elements to a fixed width. This helps with client rect measurements
+ * (i.e. that the datepicker aligns properly). Inputs have different dimensions in different
+ * browsers. e.g. in Firefox the input width is uneven, causing unexpected deviations in measuring.
+ * Note: The input should be able to shrink as on iOS the viewport width is very little but the
+ * datepicker inputs should not leave the viewport (as that throws off measuring too).
+ */
+const inputFixedWidthStyles = `
+  input {
+    width: 100%;
+    max-width: 300px;
+    box-sizing: border-box;
+  }
+`;
 
 @Component({
   template: `
@@ -1941,6 +1955,7 @@ describe('MatDatepicker', () => {
       [xPosition]="xPosition"
       [yPosition]="yPosition"></mat-datepicker>
   `,
+  styles: [inputFixedWidthStyles]
 })
 class StandardDatepicker {
   opened = false;