fix(autocomplete): prevent opening using arrow keys on readonly input

src/lib/autocomplete/autocomplete-trigger.ts

@@ -294,7 +294,7 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
 
       if (this.panelOpen || keyCode === TAB) {
         this.autocomplete._keyManager.onKeydown(event);
-      } else if (isArrowKey) {
+      } else if (isArrowKey && this._canOpen()) {
         this.openPanel();
       }
 
@@ -308,14 +308,14 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
     // We need to ensure that the input is focused, because IE will fire the `input`
     // event on focus/blur/load if the input has a placeholder. See:
     // https://connect.microsoft.com/IE/feedback/details/885747/
-    if (document.activeElement === event.target) {
+    if (this._canOpen() && document.activeElement === event.target) {
       this._onChange((event.target as HTMLInputElement).value);
       this.openPanel();
     }
   }
 
   _handleFocus(): void {
-    if (!this._element.nativeElement.readOnly) {
+    if (this._canOpen()) {
       this._attachOverlay();
       this._floatLabel(true);
     }
@@ -513,4 +513,10 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
     this.autocomplete._keyManager.setActiveItem(-1);
   }
 
+  /** Determines whether the panel can be opened. */
+  private _canOpen(): boolean {
+    const element: HTMLInputElement = this._element.nativeElement;
+    return !element.readOnly && !element.disabled;
+  }
+
 }

src/lib/autocomplete/autocomplete.spec.ts

@@ -134,6 +134,19 @@ describe('MatAutocomplete', () => {
       expect(trigger.panelOpen).toBe(false, 'Expected panel to stay closed.');
     }));
 
+    it('should not open using the arrow keys when the input is readonly', fakeAsync(() => {
+      const trigger = fixture.componentInstance.trigger;
+      input.readOnly = true;
+      fixture.detectChanges();
+
+      expect(trigger.panelOpen).toBe(false, 'Expected panel state to start out closed.');
+      dispatchKeyboardEvent(input, 'keydown', DOWN_ARROW);
+      flush();
+
+      fixture.detectChanges();
+      expect(trigger.panelOpen).toBe(false, 'Expected panel to stay closed.');
+    }));
+
     it('should open the panel programmatically', () => {
       expect(fixture.componentInstance.trigger.panelOpen)
           .toBe(false, `Expected panel state to start out closed.`);