Fix some selection-related issues in input and textarea

When setting the value of a text input, the selection is now set to the end of the new text. This also fixes textarea's initial section selection start/end to be 0.

Fixes #2787.

lib/jsdom/living/nodes/HTMLInputElement-impl.js

@@ -404,8 +404,7 @@ class HTMLInputElementImpl extends HTMLElementImpl {
         this._dirtyValue = true;
 
         if (oldValue !== this._value) {
-          this._selectionStart = 0;
-          this._selectionEnd = 0;
+          this._selectionStart = this._selectionEnd = this._getValueLength();
           this._selectionDirection = "none";
         }
         break;

lib/jsdom/living/nodes/HTMLTextAreaElement-impl.js

@@ -17,6 +17,8 @@ class HTMLTextAreaElementImpl extends HTMLElementImpl {
   constructor(globalObject, args, privateData) {
     super(globalObject, args, privateData);
 
+    this._selectionStart = this._selectionEnd = 0;
+    this._selectionDirection = "none";
     this._rawValue = "";
     this._dirtyValue = false;
 
@@ -69,12 +71,15 @@ class HTMLTextAreaElementImpl extends HTMLElementImpl {
   }
 
   set value(val) {
+    // https://html.spec.whatwg.org/multipage/form-elements.html#dom-textarea-value
+    const oldAPIValue = this._getAPIValue();
     this._rawValue = val;
     this._dirtyValue = true;
 
-    this._selectionStart = 0;
-    this._selectionEnd = 0;
-    this._selectionDirection = "none";
+    if (oldAPIValue !== this._getAPIValue()) {
+      this._selectionStart = this._selectionEnd = this._getValueLength();
+      this._selectionDirection = "none";
+    }
   }
 
   get textLength() {

test/web-platform-tests/to-run.yaml

@@ -574,6 +574,16 @@ DIR: html/semantics/forms/resetting-a-form
 
 ---
 
+DIR: html/semantics/forms/textfieldselection
+
+select-event.html: [fail, not yet implemented]
+selection-not-application-textarea.html: [fail, not yet implemented]
+selection-start-end-extra.html: [fail, not yet implemented. Shortening value by turning input type into url - fix would cause the-input-element type-change-state.html to fail.]
+textfieldselection-setRangeText.html: [fail, not yet implemented]
+textfieldselection-setSelectionRange.html: [fail, not yet implemented]
+
+---
+
 DIR: html/semantics/forms/the-button-element
 
 button-activate.html: [fail, iframe onload needs to run after synchronous script]