static navbar shakes horizontaly on leaving modal

Question

currently I have troubles with bootstrap5 and modal respectively glightbox.

when switching back from these applications, the static navbar shakes horizontaly.
the same with glightbox, additional to that the sticky footer also slides up when the screen is grayed out.

deliberate specific features: sticky footer, vertical scrollbar always on

fullscreen sample: https://codepen.io/manu_g/full/dyZOdbV

user_12352865 · Answer

Inspecting your codepen, I observed that the margin-left and margin-right of the child element of the #header increases, when the modal opens.

When you leave modal, margin-left and margin-right are returning back to their initial values, causing the shake.

The fixed position seems to be the root of cause. Thus, I would go with position: sticky instead and add some margin top in the .inner-pages.

For example,

.fixed-top {
    position: sticky;
    top: 0;
    right: 0;
    left: 0;
    z-index: 1030;
}

.inner-page {
    margin-top: 50px;
}