don't use backdrop for dialogs

[mawinter69]

The backdrop can make dialogs look really bad when there are large areas with dark color right behind the dialog (see screenshot).

Before:

After:

Testing done

Manual testing

Proposed changelog entries

• Prevent backdrop color from affecting dialogs that appear above the backdrop.

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N/A

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[timja]

/label ready-for-merge

---

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!

[StefanSpieker]

Thanks for the fix, I also noticed that today.

[janfaracik]

Could we potentially keep the backdrop filter but remove the brightness() (which needs to be removed anyway as it's breaking on HDR displays...):

  background-color: color-mix(in srgb, var(--background) 85%, transparent);
  box-shadow: var(--dialog-box-shadow), 0 0 0 100vmax rgba(23, 28, 37, 0.6);
  backdrop-filter: blur(30px);
  animation: jenkins-dialog-animate-in 0.25s cubic-bezier(0, 0.68, 0.5, 1.5);
  overflow: hidden;
  padding: $jenkins-dialog-padding 0 0 0;
  display: flex;
  flex-direction: column;
  gap: $jenkins-dialog-padding;

  &__title {
    font-size: 1.125rem;
    font-weight: 500;
    padding: 0 $jenkins-dialog-padding;
    color: var(--text-color);
  }

Something like the above - the blur is still there just drastically toned down (so we still get a little colour bleed, but nothing overtly distracting like it is now). ::backdrop has been killed in favour of the increased box-shadow as backdrop appears behind the dialog, darkening it, whereas box shadow masks as expected.

Very exaggerated example where the above still looks alright:

[mawinter69]

I would prefer that the dialog has a clear one color only background. It should not be influenced by what is behind the dialog in any way.
btw the same applies to drop down menus and tooltips

It just looks not good I think