Fix dialog background layout shift

[cyeet]

Closes https://github.com/github/copilot-productivity/issues/2608

Background

Original PR: Prevents body scroll when Dialog is open #3547

This issue is caused by the overscroll: hidden behaviour which removes the scrollbar when the dialog is shown.
As you can see, the layout noticeably shifts in the background.

current-behaviour.mov

I have explored several solutions, each with its own tradeoffs:

1. overscroll-behavior
   I tried adding overscroll-behavior: contain on the dialog and ::backdrop element but that didn’t seem to work.

2. Always showing the scrollbars
   Using overscroll-gutter: stable or overflow-y: scroll will always show the scrollbar, which leaves a blank strip down the side of the page even when the scrollbar isn’t there, which can look awkward. But this is the most stable solution, likely to work on untested layouts.

scrollbar-gutter.mov

3. Compensate for missing scrollbar
   Another avenue I tried was to add a padding or margin to the body to compensate for the missing scrollbar ala Bootstrap. It works but any fixed position components will still see a jitter as the top/right/bottom/left positioning doesn't respect padding or margin. It could be fixed programmatically by finding all fixed position elements and adding an offset calc(100vw - 100%).

Notice the background layout is unchanged.

padding-right.mov

4. Position fixed
   Finally I followed the suggestion from StackOverflow.
   It kinda works but could likely fail with other layouts.

position-fixed-solution.mov

5. Positioning inside a 100vw width parent
   I haven't tried this: https://dev.to/rashidshamloo/preventing-the-layout-shift-caused-by-scrollbars-2flp

6. Leave it
   I'm not sure if the solutions listed above is worth the problems they might caused.

References:
https://frontendmasters.com/blog/scroll-locked-dialogs/

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size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	103.83 KB (+0.13% 🔺)
packages/react/dist/browser.umd.js	104.14 KB (-0.01% 🔽)