fix(datepicker): not showing single-day comparison ranges

The logic that determines whether we show something as being a range doesn't allow for the start and end to be on the same date, because it increases the CSS complexity a lot. As a result, we aren't able to render ranges that start and end on the same day. This isn't a problem for the main range, because we style its start and end as selected which looks exactly the same as the start/end of a range, however it breaks down for the comparison range since it doesn't have the same selected styling for its start/end cells.

These changes fix the issue by introducing a special class for the case where the comparison range is for a single day. While it's not ideal, it's preferrable to complicating the CSS for the range styling.

Fixes #20100.

Author: crisbeto

src/material/datepicker/_datepicker-theme.scss

@@ -40,10 +40,18 @@ $mat-calendar-weekday-table-font-size: 11px !default;
   }
 }
 
+// Utility mixin to target cells that aren't selected. Used to make selector easier to follow.
+@mixin _mat-datepicker-unselected-cell {
+  &:not(.mat-calendar-body-selected):not(.mat-calendar-body-comparison-identical) {
+    @content;
+  }
+}
+
 @mixin mat-datepicker-color($config-or-theme) {
   $config: mat-get-color-config($config-or-theme);
   $foreground: map-get($config, foreground);
   $background: map-get($config, background);
+  $disabled-color: mat-color($foreground, disabled-text);
 
   .mat-calendar-arrow {
     border-top-color: mat-color($foreground, icon);
@@ -75,16 +83,23 @@ $mat-calendar-weekday-table-font-size: 11px !default;
     border-color: transparent;
   }
 
-  .mat-calendar-body-disabled > .mat-calendar-body-cell-content:not(.mat-calendar-body-selected),
+  .mat-calendar-body-disabled > .mat-calendar-body-cell-content {
+    @include _mat-datepicker-unselected-cell {
+      color: $disabled-color;
+    }
+  }
+
   .mat-form-field-disabled .mat-date-range-input-separator {
-    color: mat-color($foreground, disabled-text);
+    color: $disabled-color;
   }
 
   .mat-calendar-body-cell:not(.mat-calendar-body-disabled):hover,
   .cdk-keyboard-focused .mat-calendar-body-active,
   .cdk-program-focused .mat-calendar-body-active {
-    & > .mat-calendar-body-cell-content:not(.mat-calendar-body-selected) {
-      background-color: mat-color($background, hover);
+    & > .mat-calendar-body-cell-content {
+      @include _mat-datepicker-unselected-cell {
+        background-color: mat-color($background, hover);
+      }
     }
   }
 
@@ -102,24 +117,28 @@ $mat-calendar-weekday-table-font-size: 11px !default;
     }
   }
 
-  .mat-calendar-body-today:not(.mat-calendar-body-selected) {
-    // Note: though it's not text, the border is a hint about the fact that this is today's date,
-    // so we use the hint color.
-    border-color: mat-color($foreground, hint-text);
+  .mat-calendar-body-today {
+    @include _mat-datepicker-unselected-cell {
+      // Note: though it's not text, the border is a hint about the fact that this is today's date,
+      // so we use the hint color.
+      border-color: mat-color($foreground, hint-text);
+    }
   }
 
-  .mat-calendar-body-disabled > .mat-calendar-body-today:not(.mat-calendar-body-selected) {
-    $color: mat-color($foreground, hint-text);
-
-    @if (type-of($color) == color) {
-      border-color: fade-out($color, $mat-datepicker-today-fade-amount);
-    }
-    @else {
-      // If the color didn't resolve to a color value, but something like a CSS variable, we can't
-      // fade it out so we fall back to reducing the element opacity. Note that we don't use the
-      // $mat-datepicker-today-fade-amount, because hint text usually has some opacity applied
-      // to it already and we don't want them to stack on top of each other.
-      opacity: 0.5;
+  .mat-calendar-body-disabled > .mat-calendar-body-today {
+    @include _mat-datepicker-unselected-cell {
+      $color: mat-color($foreground, hint-text);
+
+      @if (type-of($color) == color) {
+        border-color: fade-out($color, $mat-datepicker-today-fade-amount);
+      }
+      @else {
+        // If the color didn't resolve to a color value, but something like a CSS variable, we can't
+        // fade it out so we fall back to reducing the element opacity. Note that we don't use the
+        // $mat-datepicker-today-fade-amount, because hint text usually has some opacity applied
+        // to it already and we don't want them to stack on top of each other.
+        opacity: 0.5;
+      }
     }
   }
 
@@ -196,6 +215,7 @@ $mat-calendar-weekday-table-font-size: 11px !default;
     background: $range-color;
   }
 
+  .mat-calendar-body-comparison-identical,
   .mat-calendar-body-in-comparison-range::before {
     background: $comparison-color;
   }
@@ -210,13 +230,14 @@ $mat-calendar-weekday-table-font-size: 11px !default;
     background: linear-gradient(to left, $range-color 50%, $comparison-color 50%);
   }
 
+  .mat-calendar-body-in-range > .mat-calendar-body-comparison-identical,
   .mat-calendar-body-in-comparison-range.mat-calendar-body-in-range::after {
     background: $overlap-color;
   }
 
+  .mat-calendar-body-comparison-identical.mat-calendar-body-selected,
   .mat-calendar-body-in-comparison-range > .mat-calendar-body-selected {
     background: $overlap-selected-color;
-
   }
 }
 

src/material/datepicker/calendar-body.html

@@ -49,13 +49,14 @@
       [class.mat-calendar-body-in-preview]="_isInPreview(item.compareValue)"
       [attr.aria-label]="item.ariaLabel"
       [attr.aria-disabled]="!item.enabled || null"
-      [attr.aria-selected]="_isSelected(item)"
+      [attr.aria-selected]="_isSelected(item.compareValue)"
       (click)="_cellClicked(item, $event)"
       [style.width]="_cellWidth"
       [style.paddingTop]="_cellPadding"
       [style.paddingBottom]="_cellPadding">
       <div class="mat-calendar-body-cell-content mat-focus-indicator"
-        [class.mat-calendar-body-selected]="_isSelected(item)"
+        [class.mat-calendar-body-selected]="_isSelected(item.compareValue)"
+        [class.mat-calendar-body-comparison-identical]="_isComparisonIdentical(item.compareValue)"
         [class.mat-calendar-body-today]="todayValue === item.compareValue">
         {{item.displayValue}}
       </div>

src/material/datepicker/calendar-body.spec.ts

@@ -566,6 +566,23 @@ describe('MatCalendarBody', () => {
       expect(cells[5].classList).toContain(previewEndClass);
     });
 
+    it('should mark a cell as being identical to the comparison range', () => {
+      testComponent.comparisonStart = testComponent.comparisonEnd = 3;
+      fixture.detectChanges();
+
+      const comparisonIdenticalCells: NodeListOf<HTMLElement> =
+          fixture.nativeElement.querySelectorAll('.mat-calendar-body-comparison-identical');
+
+      expect(comparisonIdenticalCells.length).toBe(1);
+      expect(cells[2].contains(comparisonIdenticalCells[0])).toBe(true);
+      expect(cells.some(cell => {
+        const classList = cell.classList;
+        return classList.contains(startClass) || classList.contains(inRangeClass) ||
+               classList.contains(endClass) || classList.contains(comparisonStartClass) ||
+               classList.contains(inComparisonClass) || classList.contains(comparisonEndClass);
+      })).toBe(false);
+    });
+
   });
 
 });

src/material/datepicker/calendar-body.ts

@@ -149,8 +149,8 @@ export class MatCalendarBody implements OnChanges, OnDestroy {
   }
 
   /** Returns whether a cell should be marked as selected. */
-  _isSelected(cell: MatCalendarCell) {
-    return this.startValue === cell.compareValue || this.endValue === cell.compareValue;
+  _isSelected(value: number) {
+    return this.startValue === value || this.endValue === value;
   }
 
   ngOnChanges(changes: SimpleChanges) {
@@ -270,6 +270,23 @@ export class MatCalendarBody implements OnChanges, OnDestroy {
     return isInRange(value, this.comparisonStart, this.comparisonEnd, this.isRange);
   }
 
+  /**
+   * Gets whether a value is the same as the start and end of the comparison range.
+   * For context, the functions that we use to determine whether something is the start/end of
+   * a range don't allow for the start and end to be on the same day, because we'd have to use
+   * much more specific CSS selectors to style them correctly in all scenarios. This is fine for
+   * the regular range, because when it happens, the selected styles take over and still show where
+   * the range would've been, however we don't have these selected styles for a comparison range.
+   * This function is used to apply a class that serves the same purpose as the one for selected
+   * dates, but it only applies in the context of a comparison range.
+   */
+  _isComparisonIdentical(value: number) {
+    // Note that we don't need to null check the start/end
+    // here, because the `value` will always be defined.
+    return this.comparisonStart === this.comparisonEnd && value === this.comparisonStart &&
+           value === this.comparisonEnd;
+  }
+
   /** Gets whether a value is the start of the preview range. */
   _isPreviewStart(value: number) {
     return isStart(value, this.previewStart, this.previewEnd);

tools/public_api_guard/material/datepicker.d.ts

@@ -121,6 +121,7 @@ export declare class MatCalendarBody implements OnChanges, OnDestroy {
     _isComparisonBridgeEnd(value: number, rowIndex: number, colIndex: number): boolean;
     _isComparisonBridgeStart(value: number, rowIndex: number, colIndex: number): boolean;
     _isComparisonEnd(value: number): boolean;
+    _isComparisonIdentical(value: number): boolean;
     _isComparisonStart(value: number): boolean;
     _isInComparisonRange(value: number): boolean;
     _isInPreview(value: number): boolean;
@@ -129,7 +130,7 @@ export declare class MatCalendarBody implements OnChanges, OnDestroy {
     _isPreviewStart(value: number): boolean;
     _isRangeEnd(value: number): boolean;
     _isRangeStart(value: number): boolean;
-    _isSelected(cell: MatCalendarCell): boolean;
+    _isSelected(value: number): boolean;
     ngOnChanges(changes: SimpleChanges): void;
     ngOnDestroy(): void;
     static ɵcmp: i0.ɵɵComponentDefWithMeta<MatCalendarBody, "[mat-calendar-body]", ["matCalendarBody"], { "label": "label"; "rows": "rows"; "todayValue": "todayValue"; "startValue": "startValue"; "endValue": "endValue"; "labelMinRequiredCells": "labelMinRequiredCells"; "numCols": "numCols"; "activeCell": "activeCell"; "isRange": "isRange"; "cellAspectRatio": "cellAspectRatio"; "comparisonStart": "comparisonStart"; "comparisonEnd": "comparisonEnd"; "previewStart": "previewStart"; "previewEnd": "previewEnd"; }, { "selectedValueChange": "selectedValueChange"; "previewChange": "previewChange"; }, never, never>;