HTML side-by-side mode reduces size of main content more than necessary

In testing HTML side-by-side mode via the `html_side_by_side` feature flag, I've noticed that it often reduces the size of the main content more than necessary. Depending on the size of the browser window and the content layout, this can result in content being resized when it doesn't need to be.

These gaps are not always entirely a bad thing, because they can make the visual margins to the left and right of the content more even. However reducing the content size increases the possibility of annoying layout behavior compared to when the sidebar is not in use, and it looks odd if the content gets resized when there is already enough space between the right edge of the content and the right edge of the page, to accommodate the sidebar.

**Steps to reproduce:**

1. Turn on the `html_side_by_side` feature flag
2. Go to one of the example URLs below
3. Resize the browser window to be 1200px wide (see http://howbigismybrowser.com or search the web for similar tools)
4. Load the Hypothesis client and open the sidebar

**Expected result:** Small gap between right edge of content and sidebar
**Actual result:** Big gap between right edge of content and sidebar

**Example URLs:**

1. https://pointersgonewild.com/2022/05/23/minimalism-in-programming-language-design/