fix(autosize): incorrect height with long placeholders

Long placeholders can cause the `scrollHeight` to be bigger than the actual content is. To ensure the `scrollHeight` is correct, the placeholders need to be removed temporarily.

Fixes #8013

src/lib/input/autosize.spec.ts

@@ -28,7 +28,8 @@ describe('MatTextareaAutosize', () => {
         AutosizeTextareaInATab,
         AutosizeTextAreaWithContent,
         AutosizeTextAreaWithValue,
-        AutosizeTextareaWithNgModel
+        AutosizeTextareaWithNgModel,
+        AutosizeTextareaWithLongPlaceholder
       ],
     });
 
@@ -176,6 +177,39 @@ describe('MatTextareaAutosize', () => {
       .toBe(textarea.scrollHeight, 'Expected textarea height to match its scrollHeight');
   });
 
+  it('should not calculate wrong content height due to long placeholders', () => {
+    const fixtureWithPlaceholder = TestBed.createComponent(AutosizeTextareaWithLongPlaceholder);
+    fixtureWithPlaceholder.detectChanges();
+
+    textarea = fixtureWithPlaceholder.nativeElement.querySelector('textarea');
+    autosize = fixtureWithPlaceholder.debugElement.query(
+      By.directive(MatTextareaAutosize)).injector.get<MatTextareaAutosize>(MatTextareaAutosize);
+
+    // To be able to trigger a new calculation of the height with a long placeholder, the textarea
+    // value needs to be changed.
+    textarea.value = '1';
+    autosize.resizeToFitContent();
+
+    textarea.value = '';
+    autosize.resizeToFitContent();
+
+    const heightWithLongPlaceholder = textarea.clientHeight;
+
+    fixtureWithPlaceholder.componentInstance.placeholder = 'Short';
+    fixtureWithPlaceholder.detectChanges();
+
+    // To be able to trigger a new calculation of the height with a short placeholder, the
+    // textarea value needs to be changed.
+    textarea.value = '1';
+    autosize.resizeToFitContent();
+
+    textarea.value = '';
+    autosize.resizeToFitContent();
+
+    expect(textarea.clientHeight).toBe(heightWithLongPlaceholder,
+        'Expected the textarea height to be the same with a long placeholder.');
+  });
+
   it('should resize when an associated form control value changes', fakeAsync(() => {
     const fixtureWithForms = TestBed.createComponent(AutosizeTextareaWithNgModel);
     textarea = fixtureWithForms.nativeElement.querySelector('textarea');
@@ -269,6 +303,17 @@ class AutosizeTextareaWithNgModel {
   model = '';
 }
 
+@Component({
+  template: `
+    <mat-form-field style="width: 100px">
+      <textarea matInput matTextareaAutosize [placeholder]="placeholder"></textarea>
+    </mat-form-field>`,
+  styles: [textareaStyleReset],
+})
+class AutosizeTextareaWithLongPlaceholder {
+  placeholder = 'Long Long Long Long Long Long Long Long Placeholder';
+}
+
 @Component({
   template: `
     <mat-tab-group>

src/lib/input/autosize.ts

@@ -150,14 +150,21 @@ export class MatTextareaAutosize implements AfterViewInit, DoCheck {
       return;
     }
 
+    const previousPlaceholder = textarea.placeholder;
+
     // Reset the textarea height to auto in order to shrink back to its default size.
     // Also temporarily force overflow:hidden, so scroll bars do not interfere with calculations.
+    // Long placeholders that are wider than the textarea width may lead to a bigger scrollHeight
+    // value. To ensure that the scrollHeight is not bigger than the content, the placeholders
+    // need to be removed temporarily.
     textarea.style.height = 'auto';
     textarea.style.overflow = 'hidden';
+    textarea.placeholder = '';
 
     // Use the scrollHeight to know how large the textarea *would* be if fit its entire value.
     textarea.style.height = `${textarea.scrollHeight}px`;
     textarea.style.overflow = '';
+    textarea.placeholder = previousPlaceholder;
 
     this._previousValue = value;
   }