fix(text-field): unable to undo/redo in autosized text field on firef…

…ox (#19238)

src/cdk/text-field/_text-field.scss

@@ -27,13 +27,28 @@
   // removed when measuring is complete. We use `!important` rules here to make sure user-specified
   // rules do not interfere with the measurement.
   textarea.cdk-textarea-autosize-measuring {
+    @include _cdk-textarea-autosize-measuring-base;
     height: auto !important;
     overflow: hidden !important;
-    // Having 2px top and bottom padding seems to fix a bug where Chrome gets an incorrect
-    // measurement. We just have to account for it later and subtract it off the final result.
-    padding: 2px 0 !important;
-    box-sizing: content-box !important;
   }
+
+  // Similar to the `cdk-textarea-autosize-measuring` class, but only applied on Firefox. We need
+  // to use this class, because Firefox has a bug where changing the `overflow` breaks the user's
+  // ability to undo/redo what they were typing (see #16629). This class is only scoped to Firefox,
+  // because the measurements there don't seem to be affected by the `height: 0`, whereas on other
+  // browsers they are, e.g. Chrome detects longer text and IE does't resize back to normal.
+  // Identical issue report: https://bugzilla.mozilla.org/show_bug.cgi?id=448784
+  textarea.cdk-textarea-autosize-measuring-firefox {
+    @include _cdk-textarea-autosize-measuring-base;
+    height: 0 !important;
+  }
+}
+
+@mixin _cdk-textarea-autosize-measuring-base {
+  // Having 2px top and bottom padding seems to fix a bug where Chrome gets an incorrect
+  // measurement. We just have to account for it later and subtract it off the final result.
+  padding: 2px 0 !important;
+  box-sizing: content-box !important;
 }
 
 // Used to generate UIDs for keyframes used to change the text field autofill styles.

src/cdk/text-field/autosize.ts

@@ -94,6 +94,9 @@ export class CdkTextareaAutosize implements AfterViewInit, DoCheck, OnDestroy {
   /** Used to reference correct document/window */
   protected _document?: Document;
 
+  /** Class that should be applied to the textarea while it's being measured. */
+  private _measuringClass: string;
+
   constructor(private _elementRef: ElementRef<HTMLElement>,
               private _platform: Platform,
               private _ngZone: NgZone,
@@ -102,6 +105,9 @@ export class CdkTextareaAutosize implements AfterViewInit, DoCheck, OnDestroy {
     this._document = document;
 
     this._textareaElement = this._elementRef.nativeElement as HTMLTextAreaElement;
+    this._measuringClass = _platform.FIREFOX ?
+      'cdk-textarea-autosize-measuring-firefox' :
+      'cdk-textarea-autosize-measuring';
   }
 
   /** Sets the minimum height of the textarea as determined by minRows. */
@@ -229,16 +235,16 @@ export class CdkTextareaAutosize implements AfterViewInit, DoCheck, OnDestroy {
     // 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.classList.add('cdk-textarea-autosize-measuring');
+    textarea.classList.add(this._measuringClass);
     textarea.placeholder = '';
 
-    // The cdk-textarea-autosize-measuring class includes a 2px padding to workaround an issue with
-    // Chrome, so we account for that extra space here by subtracting 4 (2px top + 2px bottom).
+    // The measuring class includes a 2px padding to workaround an issue with Chrome,
+    // so we account for that extra space here by subtracting 4 (2px top + 2px bottom).
     const height = textarea.scrollHeight - 4;
 
     // Use the scrollHeight to know how large the textarea *would* be if fit its entire value.
     textarea.style.height = `${height}px`;
-    textarea.classList.remove('cdk-textarea-autosize-measuring');
+    textarea.classList.remove(this._measuringClass);
     textarea.placeholder = placeholderText;
 
     this._ngZone.runOutsideAngular(() => {