<p dir="auto">I've created a navbar fixed to top and some content. When the modal is shown, a white 15px margin appears on the right side that pushes the content left.</p>
<p dir="auto"><a target="_blank" rel="noopener noreferrer nofollow" href="https://camo.githubusercontent.com/346a2b5848966cb9fdf3ca3235ab86fd5144f92191c15c0581be5169d6b58de6/68747470733a2f2f662e636c6f75642e6769746875622e636f6d2f6173736574732f3834323431322f313037303931312f34626330366335382d313436382d313165332d383433332d6430666464316166356134332e706e67"><img src="https://camo.githubusercontent.com/346a2b5848966cb9fdf3ca3235ab86fd5144f92191c15c0581be5169d6b58de6/68747470733a2f2f662e636c6f75642e6769746875622e636f6d2f6173736574732f3834323431322f313037303931312f34626330366335382d313436382d313165332d383433332d6430666464316166356134332e706e67" alt="modal" data-canonical-src="https://f.cloud.github.com/assets/842412/1070911/4bc06c58-1468-11e3-8433-d0fdd1af5a43.png" style="max-width: 100%;"></a></p>
<p dir="auto">It appears to be this rule:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="body.modal-open, 
.modal-open .navbar-fixed-top, 
.modal-open .navbar-fixed-bottom {
    margin-right: 15px;
}"><pre class="notranslate"><code class="notranslate">body.modal-open, 
.modal-open .navbar-fixed-top, 
.modal-open .navbar-fixed-bottom {
    margin-right: 15px;
}
</code></pre></div>
<p dir="auto">I tested it on OSX in Chrome, Firefox and Opera and it shows similar behavior in all.</p>
<p dir="auto">When launching the modal component (<a href="http://getbootstrap.com/javascript/#modals" rel="nofollow">http://getbootstrap.com/javascript/#modals</a>) the entire content will slightly move to the left on mac OS (haven't tried it on windows yet). With the active modal the scrollbar seem to disappear, while the content width still changes.</p>
<p dir="auto">You can observer the problem on the bootstrap page</p>