Why does my Image overlap my fixed top bar on hover?

I am pretty new to coding. I am writing my first Kirby Website. I have a fixed top bar and a scrollable gallery. I tried to have my images to be B/W on hover. This works, but for some reason - when hovering over the image - the image will suddenly overlap the fixed top bar. Why is this happening and what can I do?

I tried solving it with z-index but it did not work. Maybe my css is to messy? Like I said - I am just starting out and did a lot of it by trial and error.

<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title><?= $site->title() ?></title>
    <?= css('index.css') ?>


</head>


</head>
<body>
<header> 
<div class="left">  
<div class="fixed">   
<div class="logo">
        <a href="<?= $site->url() ?>"><?= $site->title() ?></a>
        <a class="sublogo"> Sound Design | Composing | Post Produktion </a>
        </div>
        </div>
        </div>
          
</header>   
<div class="flexbox"> 
  <div class="left">

      <div class="fixed">
      <nav class="menu" id="menu" >
      <?php
// get the first set of subpages which should be used
$subpages = $pages->find('projects')->children();
// create the snippet beginning with those subpages
snippet('treemenu', ['subpages' => $subpages]);
?>


        </nav>  
        <nav class="menu" id="menu2" >
 <?php foreach ($site->children()->listed() as $subpage): ?>
                <a href="<?= $subpage->url() ?>"><?= $subpage->title() ?></a>
                <?php endforeach ?>
                </nav>  
      </div>
  </div>


  <div class="right">
  <ul class="projects">
<?php foreach ($page->children()->listed() as $project): ?>
    <li>
    <div>
        <a href="<?= $project->url() ?>">
        <?= $project->image()->crop(500) ?>
        </a>
       
    </div>
</li>   
<?php endforeach ?>

</ul>  

  </div>

</div>
</body>
</body>

</body>
</html>

*,
*:after,
*:before {
  margin: 0;
  padding: 0;
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box
}


html {
    height: 100%;
    overflow: hidden;
    font-family: ui-sans-serif,system-ui,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,"Helvetica Neue",Arial,"Noto Sans",sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol","Noto Color Emoji"
}

body {
  background-color: white;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  overflow-x: hidden;
  word-wrap: break-word; 
  overflow: auto;
}
 
a {
  color: inherit;
  text-decoration: none;
  pointer-events: auto;
}

/*Helps With Images*/
img, video {
  width:100%;
  height:auto;
}

img:hover {
  filter: grayscale(100%);
  -webkit-filter: grayscale(100%);
}

li {
  list-style-type: none;
}

.logo {
  background-color: white;
  overflow: hidden;
  position: fixed;
  float: left;
  display: block;
  padding-top: 3%;
  top: 0;
  padding-bottom: 3%;
  width: 100%;
  margin: 0% 1%;
  font-size: 18px;
  font-weight: bold;
/* border-bottom: 0.125vw solid #000000; */
}

.sublogo {
  flex-basis: auto;
  margin-left: 85px;
  font-size: 18px;
  font-weight: normal;
}

.menu {
  line-height: 150%;
  font-size: 16px;
  color: black;
  
}

#menu {
  /* text-decoration: underline; */
  width: 70%;
  margin: 0% 5%;
}

#menu2 {
    width: 5%;
    margin-bottom: 30px;
    margin-left : 15px;
    position: fixed;
    bottom: 0;
}

a:hover {
  font-style: italic;
  text-decoration: underline;
}

a.logo:hover {
  font-style: normal;
  text-decoration: none;
}

a.sublogo:hover {
  font-style: normal;
  text-decoration: none;
}

.projects {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  list-style: none;
  width: 95%;
  grid-gap: 0.2rem;
  margin-top: 3.5%;
  margin-left: 11%;

}

ul.gallery {
  width: 80%;
  padding: 0% 10% 0% 10%;
  
  
}

.flexbox {
    display: flex;
    width: 100%;
    height: 100vh;
    
}

.left {
    flex-basis: auto;
    width: 12%;
    padding: 2rem;
    /* border-right: 0.125vw solid #a6a6a6; */
}

.right {
   width: 80%;
}

.fixed {
  position: fixed;
}

Solution

Why is this happening?

As per the CSS specification for filter:

A computed value of other than none results in the creation of a stacking context […]

And as per the CSS specification for the paint order of elements, elements with the same z-index within the same stacking will be painted on top of each other in "tree order" – the order they appear in the HTML code. Since the images appear later than the top bar, the images will overlay the top bar.

what can I do?

Increase the z-index of the top bar element:

.fixed {
  …
  z-index: 1;
}

*,
*:after,
*:before {
  margin: 0;
  padding: 0;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box
}

html {
  height: 100%;
  overflow: hidden;
  font-family: ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji"
}

body {
  background-color: white;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  overflow-x: hidden;
  word-wrap: break-word;
  overflow: auto;
}

a {
  color: inherit;
  text-decoration: none;
  pointer-events: auto;
}


/*Helps With Images*/

img,
video {
  width: 100%;
  height: auto;
}

img:hover {
  filter: grayscale(100%);
  -webkit-filter: grayscale(100%);
}

li {
  list-style-type: none;
}

.logo {
  background-color: white;
  overflow: hidden;
  position: fixed;
  float: left;
  display: block;
  padding-top: 3%;
  top: 0;
  padding-bottom: 3%;
  width: 100%;
  margin: 0% 1%;
  font-size: 18px;
  font-weight: bold;
  /* border-bottom: 0.125vw solid #000000; */
}

.sublogo {
  flex-basis: auto;
  margin-left: 85px;
  font-size: 18px;
  font-weight: normal;
}

.menu {
  line-height: 150%;
  font-size: 16px;
  color: black;
}

#menu {
  /* text-decoration: underline; */
  width: 70%;
  margin: 0% 5%;
}

#menu2 {
  width: 5%;
  margin-bottom: 30px;
  margin-left: 15px;
  position: fixed;
  bottom: 0;
}

a:hover {
  font-style: italic;
  text-decoration: underline;
}

a.logo:hover {
  font-style: normal;
  text-decoration: none;
}

a.sublogo:hover {
  font-style: normal;
  text-decoration: none;
}

.projects {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  list-style: none;
  width: 95%;
  grid-gap: 0.2rem;
  margin-top: 3.5%;
  margin-left: 11%;
}

ul.gallery {
  width: 80%;
  padding: 0% 10% 0% 10%;
}

.flexbox {
  display: flex;
  width: 100%;
  height: 100vh;
}

.left {
  flex-basis: auto;
  width: 12%;
  padding: 2rem;
  /* border-right: 0.125vw solid #a6a6a6; */
}

.right {
  width: 80%;
}

.fixed {
  position: fixed;
  z-index: 1;
}

<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
</head>

<body>
  <header>
    <div class="left">
      <div class="fixed">
        <div class="logo">
          <a href="">&lt;?= $site->title() ?></a>
          <a class="sublogo"> Sound Design | Composing | Post Produktion </a>
        </div>
      </div>
    </div>

  </header>
  <div class="flexbox">
    <div class="left">

      <div class="fixed">
        <nav class="menu" id="menu">
        </nav>
        <nav class="menu" id="menu2">
          <a href="">&lt;?= $subpage->title() ?></a>
        </nav>
      </div>
    </div>


    <div class="right">
      <ul class="projects">
        <li>
          <div>
            <a href="<?= $project->url() ?>">
              <img src="https://picsum.photos/500/500" width="500" height="500"/>
            </a>
            <a href="<?= $project->url() ?>">
              <img src="https://picsum.photos/500/500" width="500" height="500"/>
            </a>
            <a href="<?= $project->url() ?>">
              <img src="https://picsum.photos/500/500" width="500" height="500"/>
            </a>
            <a href="<?= $project->url() ?>">
              <img src="https://picsum.photos/500/500" width="500" height="500"/>
            </a>
          </div>
        </li>
      </ul>
    </div>
  </div>
</body>
</body>

</body>

</html>