fix(material/form-field): hiding a label after it has been (#29461)

shown leaves a blank space

There is a method _shouldLabelFloat that determines if the label
should float. A check was added `_hasFloatingLabel` to see if a
floating label exists before deciding whether the label should
float. Examples were added at the end of the input-demo file,
where you can see inputs without labels (both fixed and dynamic).
Removing the solution also allows you to simulate the described
error. Unit tests were added to validate the solution.

Fixes #29401

(cherry picked from commit 13aef8c)

src/dev-app/input/input-demo.html

@@ -880,6 +880,41 @@ <h4>Custom control</h4>
   </mat-card-content>
 </mat-card>
 
+<mat-card class="demo-card demo-basic">
+  <mat-toolbar color="primary">Without label</mat-toolbar>
+  <mat-card-content>
+    <h4>Label removed</h4>
+    <p>
+    <mat-form-field appearance="outline">
+      @if (hasLabel$ | async){
+        <mat-label>My input</mat-label>
+      }
+      <input matInput type="text" />
+    </mat-form-field>
+    </p>
+    <p>
+    <mat-form-field>
+      @if (hasLabel$ | async){
+        <mat-label>My input</mat-label>
+      }
+      <input matInput type="text" />
+    </mat-form-field>
+    </p>
+    <h4>No defined label</h4>
+    <p>
+    <mat-form-field appearance="outline">
+      <input matInput type="text"/>
+    </mat-form-field>
+    </p>
+    <p>
+      <mat-form-field>
+        <input matInput type="text"/>
+      </mat-form-field>
+    </p>
+  </mat-card-content>
+</mat-card>
+
+
 <mat-card class="demo-card demo-basic">
   <mat-card-content>
     <button (click)="showHidden = !showHidden">Show/hide hidden form-field</button>

src/dev-app/input/input-demo.ts

@@ -29,6 +29,7 @@ import {MatIconModule} from '@angular/material/icon';
 import {MatTabsModule} from '@angular/material/tabs';
 import {MatToolbarModule} from '@angular/material/toolbar';
 import {MatTooltipModule} from '@angular/material/tooltip';
+import {BehaviorSubject} from 'rxjs';
 
 let max = 5;
 
@@ -100,8 +101,13 @@ export class InputDemo {
   fillAppearance: string;
   outlineAppearance: string;
 
+  hasLabel$ = new BehaviorSubject(true);
+
   constructor() {
-    setTimeout(() => this.delayedFormControl.setValue('hello'), 100);
+    setTimeout(() => {
+      this.delayedFormControl.setValue('hello');
+      this.hasLabel$.next(false);
+    }, 100);
   }
 
   addABunch(n: number) {

src/material/form-field/form-field.ts

@@ -555,7 +555,10 @@ export class MatFormField
 
   _hasFloatingLabel = computed(() => !!this._labelChild());
 
-  _shouldLabelFloat() {
+  _shouldLabelFloat(): boolean {
+    if (!this._hasFloatingLabel()) {
+      return false;
+    }
     return this._control.shouldLabelFloat || this._shouldAlwaysFloat();
   }
 

src/material/input/input.spec.ts

@@ -157,7 +157,8 @@ describe('MatMdcInput without forms', () => {
     fixture.detectChanges();
 
     expect(formField._control.empty).toBe(false);
-    expect(formField._shouldLabelFloat()).toBe(true);
+    // should not float label if there is no label
+    expect(formField._shouldLabelFloat()).toBe(false);
   }));
 
   it('should not be empty when the value set before view init', fakeAsync(() => {
@@ -1531,6 +1532,62 @@ describe('MatFormField default options', () => {
     ).toBe(true);
   });
 });
+describe('MatFormField without label', () => {
+  it('should not float the label when no label is defined.', () => {
+    let fixture = createComponent(MatInputWithoutDefinedLabel);
+    fixture.detectChanges();
+
+    const inputEl = fixture.debugElement.query(By.css('input'))!;
+    const formField = fixture.debugElement.query(By.directive(MatFormField))!
+      .componentInstance as MatFormField;
+
+    // Update the value of the input and set focus.
+    inputEl.nativeElement.value = 'Text';
+    fixture.detectChanges();
+
+    // should not float label if there is no label
+    expect(formField._shouldLabelFloat()).toBe(false);
+  });
+
+  it('should not float the label when the label is removed after it has been shown', () => {
+    let fixture = createComponent(MatInputWithCondictionalLabel);
+    fixture.detectChanges();
+    const inputEl = fixture.debugElement.query(By.css('input'))!;
+    const formField = fixture.debugElement.query(By.directive(MatFormField))!
+      .componentInstance as MatFormField;
+
+    // initially, label is present
+    expect(fixture.nativeElement.querySelector('label')).not.toBeNull();
+
+    // removing label after it has been shown
+    fixture.componentInstance.hasLabel = false;
+    inputEl.nativeElement.value = 'Text';
+    fixture.changeDetectorRef.markForCheck();
+    fixture.detectChanges();
+
+    // now expected to not have a label
+    expect(fixture.nativeElement.querySelector('label')).toBeNull();
+    // should not float label since there is no label
+    expect(formField._shouldLabelFloat()).toBe(false);
+  });
+
+  it('should float the label when the label is not removed', () => {
+    let fixture = createComponent(MatInputWithCondictionalLabel);
+    fixture.detectChanges();
+    const inputEl = fixture.debugElement.query(By.css('input'))!;
+    const formField = fixture.debugElement.query(By.directive(MatFormField))!
+      .componentInstance as MatFormField;
+
+    inputEl.nativeElement.value = 'Text';
+    fixture.changeDetectorRef.markForCheck();
+    fixture.detectChanges();
+
+    // Expected to have a label
+    expect(fixture.nativeElement.querySelector('label')).not.toBeNull();
+    // should float label since there is a label
+    expect(formField._shouldLabelFloat()).toBe(true);
+  });
+});
 
 function configureTestingModule(
   component: Type<any>,
@@ -1787,6 +1844,28 @@ class MatInputWithDynamicLabel {
   shouldFloat: 'always' | 'auto' = 'always';
 }
 
+@Component({
+  template: `
+    <mat-form-field>
+      <input matInput placeholder="Label">
+    </mat-form-field>
+  `,
+})
+class MatInputWithoutDefinedLabel {}
+
+@Component({
+  template: `
+    <mat-form-field>
+      @if (hasLabel) {
+        <mat-label>Label</mat-label>
+      }
+      <input matInput>
+    </mat-form-field>`,
+})
+class MatInputWithCondictionalLabel {
+  hasLabel = true;
+}
+
 @Component({
   template: `
     <mat-form-field>