もしかしたら、これらがあなたの欲しいH5ソフトキーボード対応ソリューションかもしれません（要約）。

// judge device type
var judgeDeviceType = function () {
  var ua = window.navigator.userAgent.toLocaleLowerCase();
  var isIOS = /iphone|ipad|ipod/.test(ua);
  var isAndroid = /android/.test(ua);

  return {
    isIOS: isIOS,
    isAndroid: isAndroid
  }
}()

// Listen to the soft keyboard pop-up and pop-down events of the input box
function listenKeybord($input) {
  if (judgeDeviceType.isIOS) {
    // IOS keyboard popup: IOS and Android input boxes get focus on keyboard popup
    $input.addEventListener('focus', function () {
      console.log('IOS keyboard is popping up!') ;
      // IOS keyboard pops up and then operates
    }, false)

    // IOS keyboard is tucked away: If IOS clicks outside the input box or clicks the tuck button, the input box will lose focus and the keyboard will be tucked away.
    $input.addEventListener('blur', () => {
      console.log('IOS keyboard is stowed!') ;
      // IOS keyboard is put away and operated
    })
  }

  // Andriod keyboard put away: Andriod keyboard popped up or put away the page height will change, as a basis to know the keyboard put away
  if (judgeDeviceType.isAndroid) {
    var originHeight = document.documentElement.clientHeight || document.body.clientHeight;

    window.addEventListener('resize', function () {
      var resizeHeight = document.documentElement.clientHeight || document.body.clientHeight;
      if (originHeight < resizeHeight) {
        console.log('Android keyboard put away!') ;
        // Android keyboard is put away and operated
      } else {
        console.log('Android keyboard popped up!') ;
        // Android keyboard pops up and then operates
      }

      originHeight = resizeHeight;
    }, false)
  }
}

var $inputs = document.querySelectorAll('.input');

for (var i = 0; i < $inputs.length; i++) {
  listenKeybord($inputs[i]);
}

// Get the focus element to scroll to the viewable area
function activeElementScrollIntoView(activeElement, delay) {
  var editable = activeElement.getAttribute('contenteditable')

  // The input box, textarea or rich text does not scroll the element to the viewable area after getting focus
  if (activeElement.tagName == 'INPUT' || activeElement.tagName == 'TEXTAREA' || editable === '' || editable) {
    setTimeout(function () {
      activeElement.scrollIntoView();
    }, delay)
  }
}

// ...
// Android keyboard operation after popup
activeElementScrollIntoView($input, 1000);
// ...

<p>Please enter a cell phone number</p>
<input type="tel" novalidate="novalidate" pattern="[0-9]*" class="input">

console.log('IOS keyboard is put away!') ;

// WeChat browser version 6.7.4 + IOS12 will appear after the keyboard is put away, the view is topped up and not down
var wechatInfo = window.navigator.userAgent.match(/MicroMessenger\/([\d\.] +)/i);
if (!wechatInfo) return;

var wechatVersion = wechatInfo[1];
var version = (navigator.appVersion).match(/OS (\d+)_(\d+)_? (\d+)? /);

if (+wechatVersion.replace(/\. /g, '') >= 674 && +version[1] >= 12) {
  window.scrollTo(0, Math.max(document.body.clientHeight, document.documentElement.clientHeight));
}

<div class="chat__content">
  <div>
    <p> Some chat content1</p>
  </div>
  <! -- omitting several thousand lines of chat -- >
</div>
<div class="input__content">
  <div class="input" contenteditable="true"></div>
  <button>send</button>
</div>

/* Omit some styles */
.chat__content {
  height: calc(100% - 40px);
  margin-bottom: 40px;
  overflow-y: auto;
  overflow-x: hidden;
}

.input__content {
  display: flex;
  height: 40px;
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  align-items: center;
}
/* Omit some styles */

console.log('Android keyboard is popping up!') ;
activeElementScrollIntoView($input, 1000);

// Andriod keyboard tucked away: The Andriod keyboard will change in height when it is popped up or tucked away, and this is the basis for knowing that the keyboard is tucked away
if (judgeDeviceType.isAndroid) {
  var originHeight = document.documentElement.clientHeight || document.body.clientHeight;

  window.addEventListener('resize', function () {
    var resizeHeight = document.documentElement.clientHeight || document.body.clientHeight;
    if (originHeight < resizeHeight) {
      console.log('Android keyboard put away!') ;

      // Fix the problem that the input box is still blocked by the input method under Xiaomi browser
      if (judgeDeviceType.isMiuiBrowser) {
        document.body.style.marginBottom = '0px';
      }
    } else {
      console.log('Android keyboard is popping up!') ;

      // Fix the problem that the input box is still blocked by the input method in Xiaomi browser
      if (judgeDeviceType.isMiuiBrowser) {
        document.body.style.marginBottom = '40px';
      }
      activeElementScrollIntoView($input, 1000);
    }

    originHeight = resizeHeight;
  }, false)
}