After some painful debugging session, I finally found out what was wrong and I believe it's completely wrong behaviour by QT here.
It's when we used a `fieldset` element, we got the layout completely destroyed in Fusion add-in context.
Here's a simple step by step repro:
- create a basic add-in that loads an HTML page
- in the body of the HTML page, put the following
```
<fieldset class="test-block">
<div><input type="text"></div>
<div><input type="text"></div>
<div><input type="text"></div>
<div><input type="text"></div>
<div><input type="text"></div>
<div><input type="text"></div>
<div><input type="text"></div>
<div><input type="text"></div>
<div><input type="text"></div>
<div><input type="text"></div>
<div><input type="text"></div>
</fieldset>
```
- add a class in the css with the following
```
.test-block{
flex: 1;
display: grid;
$min-column-width: 250px;
grid-template-columns: repeat(auto-fit, minmax($min-column-width, 1fr));
grid-gap: 1rem;
}
```
The layout will be in a broken state (see screenshot where it's all vertical).
Now replace the `fieldset` tag with a `div` for example, and the layout should be correct (see screenshot where all elements are spaced up horizontaly as expected)
