I fixed the issue by adding position:fixed to the body when opening a modal. Hope this will help you.

Personally, position: fixed scroll to top automatically. Quite annoying !

To avoid penalizing other devices and versions I apply this fix only to the appropriate versions of iOS.

**VERSION 1 - All modals fix**

For the javascript/jQuery

$(document).ready(function() {

    // Detect ios 11_x_x affected  
    // NEED TO BE UPDATED if new versions are affected
    var ua = navigator.userAgent,
    iOS = /iPad|iPhone|iPod/.test(ua),
    iOS11 = /OS 11_0|OS 11_1|OS 11_2/.test(ua);

    // ios 11 bug caret position
    if ( iOS && iOS11 ) {

        // Add CSS class to body
        $("body").addClass("iosBugFixCaret");

    }

});

For the CSS

/* Apply CSS to iOS affected versions only */
body.iosBugFixCaret.modal-open { position: fixed; width: 100%; }

**VERSION 2 - Selected modals only**

I modified the function to fire only for selected modals with a class .inputModal

Only the modals with inputs should be impacted to avoid the scroll to top.

For the javascript/jQuery

$(document).ready(function() {

    // Detect ios 11_x_x affected
    // NEED TO BE UPDATED if new versions are affected 
    (function iOS_CaretBug() {

        var ua = navigator.userAgent,
        scrollTopPosition,
        iOS = /iPad|iPhone|iPod/.test(ua),
        iOS11 = /OS 11_0|OS 11_1|OS 11_2/.test(ua);

        // ios 11 bug caret position
        if ( iOS && iOS11 ) {

            $(document.body).on('show.bs.modal', function(e) {
                if ( $(e.target).hasClass('inputModal') ) {
                    // Get scroll position before moving top
                    scrollTopPosition = $(document).scrollTop();

                    // Add CSS to body "position: fixed"
                    $("body").addClass("iosBugFixCaret");
                }
            });

            $(document.body).on('hide.bs.modal', function(e) {
                if ( $(e.target).hasClass('inputModal') ) {         
                    // Remove CSS to body "position: fixed"
                    $("body").removeClass("iosBugFixCaret");

                    //Go back to initial position in document
                    $(document).scrollTop(scrollTopPosition);
                }
            });

        }
    })();
});

For the CSS

/* Apply CSS to iOS affected versions only */
body.iosBugFixCaret.modal-open { position: fixed; width: 100%; }

For the HTML Add the class inputModal to the modal

<div class="modal fade inputModal" tabindex="-1" role="dialog">
    ...
</div>

Nota bene The javascript function is now self-invoking

**UPDATE iOS 11.3 - Bug corrected **

As of iOS 11.3, the bug has been corrected. There is no need to test for OS 11_ in iOS11 = /OS 11_0|OS 11_1|OS 11_2/.test(ua);

But be careful as iOS 11.2 is still widely used (as of April 2018). See

stat 1

stat 2

This issue goes beyond Bootstrap, and beyond just Safari. It is a full display bug in iOS 11 that occurs in all browsers. The fix above does not fix this issue in all instances.

The bug is reported in detail here:

https://medium.com/@eirik.luka/how-to-fix-the-ios-11-input-element-in-fixed-modals-bug-aaf66c7ba3f8

Supposedly they already reported it to Apple as a bug.

Frustrating bug, thanks for identifying it. Otherwise, I would be banging my iphone or my head against the wall.

The simplest fix is (1 line of code change):

Just add the following CSS to the html or to an external css file.

<style type="text/css">
.modal-open { position: fixed; }
</style>

Here is a full working example:

.modal-open { position: fixed; }

<link href="https://getbootstrap.com/docs/3.3/dist/css/bootstrap.min.css" rel="stylesheet">

<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModal" data-whatever="@mdo">Open modal for @mdo</button>
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModal" data-whatever="@fat">Open modal for @fat</button>
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModal" data-whatever="@getbootstrap">Open modal for @getbootstrap</button>
...more buttons...

<div class="modal fade" id="exampleModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h4 class="modal-title" id="exampleModalLabel">New message</h4>
      </div>
      <div class="modal-body">
        <form>
          <div class="form-group">
            <label for="recipient-name" class="control-label">Recipient:</label>
            <input type="text" class="form-control" id="recipient-name">
          </div>
          <div class="form-group">
            <label for="message-text" class="control-label">Message:</label>
            <textarea class="form-control" id="message-text"></textarea>
          </div>
        </form>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Send message</button>
      </div>
    </div>
  </div>
</div>

    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
    <script src="https://getbootstrap.com/docs/3.3/dist/js/bootstrap.min.js"></script>

I submitted an issue here: https://github.com/twbs/bootstrap/issues/24059

Easiest/cleanest solution:

body.modal-open { position: fixed; width: 100%; }

Add position: fixed; to body when modal is open.

$(document).ready(function($){
    $("#myBtn").click(function(){
        $("#myModal").modal("show");
    });
    $("#myModal").on('show.bs.modal', function () {
        $('body').addClass('body-fixed');
    });
    $("#myModal").on('hide.bs.modal', function () {
        $('body').removeClass('body-fixed');
    });
});

.body-fixed {
    position: fixed;
    width: 100%;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/js/bootstrap.min.js"></script>
<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>

<button type="button" class="btn btn-info btn-lg" id="myBtn">Open Modal</button>

<!-- Modal -->
<div class="modal fade" id="myModal" role="dialog">
 <div class="modal-dialog">
  <div class="modal-content">
   <div class="modal-header">
    <button type="button" class="close" data-dismiss="modal">&times;</button>
    <h4 class="modal-title">Form</h4>
   </div>
   <div class="modal-body">
    <div class="form-group">
     <label class="control-label">Input #1</label>
     <input type="text" class="form-control">
    </div>
    <div class="form-group">
     <label class="control-label">Input #2</label>
     <input type="text" class="form-control">
    </div>
    <div class="form-group">
     <label class="control-label">Input #3</label>
     <input type="text" class="form-control">
    </div>
    <div class="form-group">
     <label class="control-label">Input #4</label>
     <input type="text" class="form-control">
    </div>
   </div>
   <div class="modal-footer">
    <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
   </div>
  </div>
 </div>
</div>

As previously mentioned: setting the style.position property of body to fixed solves the iOS cursor misplacement issue.

However, this gain comes at the cost of being forcibly scrolled to the top of the page.

Fortunately, this new UX problem can be negated without much overhead by leveraging HTMLElement.style and window.scrollTo().

The basic gist is to counteract the scroll to top by manipulating the body element's style.top when mounting. This is done using the YOffset value captured by the ygap variable.

From there it's simply a matter of resetting the body's style.top to 0 and reframing the user's view using window.scrollTo(0, ygap) when dismounting.

See below for a practical example.

// Global Variables (Manage Globally In Scope).
const body = document.querySelector('body') // Body.
let ygap = 0 // Y Offset.


// On Mount (Call When Mounting).
const onModalMount = () => {

  // Y Gap.
  ygap = window.pageYOffset || document.documentElement.scrollTop

  // Fix Body.
  body.style.position = 'fixed'

  // Apply Y Offset To Body Top.
  body.style.top = `${-ygap}px`

}


// On Dismount (Call When Dismounting).
const onModalDismount = () => {

  // Unfix Body.
  body.style.position = 'relative'

  // Reset Top Offset.
  body.style.top = '0'

  // Reset Scroll.
  window.scrollTo(0, ygap)

}

Have you try viewport-fit=cover for the meta viewport.

Look at this : https://ayogo.com/blog/ios11-viewport/

Override modal css and change its position from fixed to absolute

.modal {
position: absolute !important;
}