将“textarea”高度保持为“100%”不起作用

.popup {
  border: 1px solid red;
  background: rgba(000, 000, 000, 0.4);
  position: fixed;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  display: flex;
  align-items: center;
  justify-content: center;
}

form,
fieldset {
  padding: 0;
  margin: 0;
  border: 0;
}

form {
  display: flex;
  flex-wrap: wrap;
  border: 1px dotted gray;
  width: 25rem;
}

fieldset {
  width: 50%;
  background: #333;
  padding: 0.5em;
  margin: 0;
  box-sizing: border-box;
}

form label {
  display: block;
  width: 100%;
  margin-bottom: 1rem;
}

form label input,
form label textarea {
  width: 100%;
  background: white;
  border: 1px solid gray;
}

textarea {
  border: 1px solid blue;
  width: 100%;
  height: 100%;
  box-sizing: border-box;
}

.submit-form {
  flex-basis: 100%;
  text-align: right;
  background: tan;
}

<div>
  <div>
    <form>
      <fieldset>
        <label htmlFor="name"><input type="text" name="name" /></label>
        <label htmlFor="email"><input type="email" name="email" /></label>
        <label htmlFor="phone"> 
          <input type="tel" name="phone" 
            autoComplete="off"
            placeholder="Phone"/> </label>
      </fieldset>
      <fieldset>
        <label htmlFor="message">
          <textarea placeholder="Message" 
            autoComplete="off"
            name="message"></textarea></label>
      </fieldset>
      <div>
        <button>Submit</button>
      </div>
    </form>
  </div>
</div>

<div>
  <div>
    <form>
      <fieldset>
        <label htmlFor="name"><input type="text" name="name" /></label>
        <label htmlFor="email"><input type="email" name="email" /></label>
        <label htmlFor="phone"> 
          <input type="tel" name="phone" 
            autoComplete="off"
            placeholder="Phone"/> </label>
      </fieldset>
      <fieldset>
        <label htmlFor="message">
          <textarea placeholder="Message" 
            autoComplete="off"
            name="message"></textarea></label>
      </fieldset>
      <div>
        <button>Submit</button>
      </div>
    </form>
  </div>
</div>

回答

.popup {
  border: 1px solid red;
  background: rgba(000, 000, 000, 0.4);
  position: fixed;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  display: flex;
  align-items: center;
  justify-content: center;
}

form,
fieldset {
  padding: 0;
  margin: 0;
  border: 0;
}

form {
  display: flex;
  flex-wrap: wrap;
  border: 1px dotted gray;
  width: 25rem;
}

fieldset {
  width: 50%;
  background: #333;
  padding: 0.5em;
  margin: 0;
  box-sizing: border-box;
}

form label {
  display: block;
  width: 100%;
  margin-bottom: 1rem;
}

form label input,
form label textarea {
  width: 100%;
  background: white;
  border: 1px solid gray;
}

textarea {
  border: 1px solid blue;
  width: 100%;
  height: 100%;
  box-sizing: border-box;
}

.submit-form {
  flex-basis: 100%;
  text-align: right;
  background: tan;
}

.message {
  height: 100%
}