fix(text-field): autosize textarea not resizing on minRows decrease

src/cdk/text-field/autosize.spec.ts

@@ -110,6 +110,22 @@ describe('CdkTextareaAutosize', () => {
         .toBeGreaterThan(previousMaxHeight, 'Expected increased max-height with maxRows increase.');
   });
 
+  it('should reduce textarea height when minHeight decreases', () => {
+    expect(textarea.style.minHeight).toBeFalsy();
+
+    fixture.componentInstance.minRows = 6;
+    fixture.detectChanges();
+
+    expect(textarea.style.minHeight).toBeDefined('Expected a min-height to be set via minRows.');
+
+    let previousHeight = parseInt(textarea.style.height!);
+    fixture.componentInstance.minRows = 3;
+    fixture.detectChanges();
+
+    expect(parseInt(textarea.style.height!))
+        .toBeLessThan(previousHeight, 'Expected decreased height with minRows decrease.');
+  });
+
   it('should export the cdkAutosize reference', () => {
     expect(fixture.componentInstance.autosize).toBeTruthy();
     expect(fixture.componentInstance.autosize.resizeToFitContent).toBeTruthy();
@@ -274,9 +290,7 @@ const textareaStyleReset = `
 @Component({
   template: `
     <textarea cdkTextareaAutosize [cdkAutosizeMinRows]="minRows" [cdkAutosizeMaxRows]="maxRows"
-        #autosize="cdkTextareaAutosize">
-      {{content}}
-    </textarea>`,
+        #autosize="cdkTextareaAutosize">{{content}}</textarea>`,
   styles: [textareaStyleReset],
 })
 class AutosizeTextAreaWithContent {

src/cdk/text-field/autosize.ts

@@ -35,14 +35,21 @@ import {fromEvent, Subject} from 'rxjs';
 })
 export class CdkTextareaAutosize implements AfterViewInit, DoCheck, OnDestroy {
   /** Keep track of the previous textarea value to avoid resizing when the value hasn't changed. */
-  private _previousValue: string;
+  private _previousValue?: string;
   private _initialHeight: string | null;
   private readonly _destroyed = new Subject<void>();
 
   private _minRows: number;
   private _maxRows: number;
   private _enabled: boolean = true;
 
+  /**
+   * Value of minRows as of last resize. If the minRows has decreased, the
+   * height of the textarea needs to be recomputed to reflect the new minimum. The maxHeight
+   * does not have the same problem because it does not affect the textarea's scrollHeight.
+   */
+  private _previousMinRows: number = -1;
+
   private _textareaElement: HTMLTextAreaElement;
 
   /** Minimum amount of rows in the textarea. */
@@ -195,8 +202,8 @@ export class CdkTextareaAutosize implements AfterViewInit, DoCheck, OnDestroy {
     const textarea = this._elementRef.nativeElement as HTMLTextAreaElement;
     const value = textarea.value;
 
-    // Only resize of the value changed since these calculations can be expensive.
-    if (value === this._previousValue && !force) {
+    // Only resize if the value or minRows have changed since these calculations can be expensive.
+    if (!force && this._minRows === this._previousMinRows && value === this._previousValue) {
       return;
     }
 
@@ -238,6 +245,7 @@ export class CdkTextareaAutosize implements AfterViewInit, DoCheck, OnDestroy {
     }
 
     this._previousValue = value;
+    this._previousMinRows = this._minRows;
   }
 
   /**

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

@@ -543,6 +543,17 @@ <h4>Textarea</h4>
     <h3>Regular &lt;textarea&gt;</h3>
     <textarea class="demo-textarea" cdkTextareaAutosize cdkAutosizeMaxRows="10"></textarea>
 
+    <h3>Regular &lt;textarea&gt; with maxRows and minRows</h3>
+    <div>
+      <label>minRows<input type="number" #minRows class="demo-rows" (input)="1+1"></label> &nbsp;
+      <label>maxRows<input type="number" #maxRows class="demo-rows" (input)="1+1"></label>
+    </div>
+    <textarea class="demo-textarea"
+        cdkTextareaAutosize
+        [cdkAutosizeMinRows]="minRows.value"
+        [cdkAutosizeMaxRows]="maxRows.value"></textarea>
+    <button type="button" (click)="minRows.value = minRows.value - 1">Decrement minRows</button>
+
     <h3>&lt;textarea&gt; with mat-form-field</h3>
     <div>
       <mat-form-field>

src/demo-app/input/input-demo.scss

@@ -35,3 +35,7 @@
 .demo-custom-autofill-style {
   @include cdk-text-field-autofill-color(transparent, red);
 }
+
+.demo-rows {
+  width: 30px;
+}