Flexbox: Combine one full-height column with following rows (two-column-layout) - html

Question

I'm working on this for hours but can not achieve a working solution. Basically, I want to combine a full-height column (left) with following rows (right) in a two-column-layout with CSS Flexbox.
The full-height column should match the height of the parent's div (.grid-main > main), which is a grid-layout on the other hand.
Screenshot of the issue:
So the yellow boxes should be next to the full-height column and not continue beneath it.
The column should match the height of .grid-main (which could be achieved with flex-direction: column, but all following divs are also listed in a column then).
HTML-Structure:
<div class="grid-main">
<header>...</header>
<nav>...</nav>
<main>
<div class="col-reports"></div>
<div class="classes"></div>
<div class="classes"></div>
<div class="classes"></div>
<div class="classes"></div>
...
</main>
<footer>...</footer>
</div>
CSS:
.grid-main {
display: grid;
height: 98vh;
width: 98vw;
overflow: hidden;
position: relative;
overflow: -moz-hidden-unscrollable;
grid-template-columns: 175px 1fr 1fr;
grid-template-rows: 0.1fr 1fr 0.05fr;
grid-template-areas:
"header header header"
"nav main main"
"footer footer footer";
animation: fadeIn 1s;
transition: 1s all ease-in-out;
}
.grid-main > main {
display: flex;
flex-direction: row;
flex-wrap: wrap;
align-items: flex-start;
place-content: flex-start leftjustify-content: flex-start space-evenly;
grid-area: main;
position: relative;
overflow-y: auto;
background: var(--color-accent-main);
}
.col-reports {
flex: 1;
height: 100%;
position: relative;
max-width: 250px;
padding: 20px;
background: rgba(204,204,204,.7);
box-shadow: 1px 1px 25px inset rgb(179, 179, 179);
font-size: .8rem;
font-weight: bold;
line-height: 200%;
}
.classes {
font-family: 'Lora';
line-height: 2rem;
background: #f6efe0;
border-radius: 5px;
padding: 20px;
margin: 10px;
border: 5px solid #eee9dd;
box-shadow: 0 0 5px rgba(0, 0, 0, 0.07);
animation: fadeInUp 1s;
}
Visualized the issue:

Answer 1

I solved the issue basically with the following code. So the hint from #Paulie_D was quite helpful, although I wanted to avoid Grid for this basic layouting.
.grid-reports {
display: grid; /* Grid in Grid */
height: 100%; /* In contrast to Flexbox, Grid can be used to assign height: 100% to a container.
It stretches to the parent container even if the viewport changes. Flexbox does not work with height actually.
Unless you use flex-direction: column; to the parent container. */
grid-template-rows: 1fr; /* optional */
grid-template-columns: 1fr 6fr;
grid-area: main; /* referes to Main in .grid-main */
}
.col-students {
grid-row: 1; /* .col-students must only be in the first row of .grid-reports to stretch */
grid-column: 1; /* Only the first column of .grid-reports */
max-width: 250px;
padding: 20px;
background: rgba(204,204,204,.7);
box-shadow: 1px 1px 25px inset rgb(179, 179, 179);
font-size: .8rem;
font-weight: bold;
line-height: 200%;
}
.col-reports {
grid-column: 2; /* All childs of .col-reports and itself are in the second column of .grid-reports
Note: grid-rows are automatically assigned, because they are not adressed any further */
display: flex; /* .col-reports becomes now a Flexbox to align its child items (box). */
flex-wrap: wrap; /* All child elements automatically break */
flex-direction: row; /* All child elements flow in a row */
align-self: flex-start; /* To align items next to each other and at start of Flex */
padding: 10px;
}
.col-reports .box { /* Further definitions of the element in .grid-main */
flex-basis: 45%; /* So the flex-basis in this case relates to the .col-reports
and not to the whole .grid-reports which makes sense and is desired behaviour.
45% means that 45% of .col-reports are used, which is in the second column. */
/*Note: This needs to be assigned as 100% in the mobile section, because elements
must be placed under each other for mobile devices. */
}