Fixes bug with whitespace on DataTable when merge_duplicate_header=True

[leeagustin]

Fixes #2870.

Before:

After:

I'm not sure how the DataTable class works exactly, but by inspecting the DOM, I was able to pinpoint that the issue is with the following:

I think what's happening is the following: inside the table dash-fixed-column, the non-header columns are being resized to the same widths as the header columns from the dash-fixed-content table. Since there are more non-header columns than header ones, the non-header columns overflow to the right. Hence, the solution would be to resize the non-header columns to the width of the non-header columns from dash-fixed-content (and not the header columns).

Before adding or modifying a test, I wanted to check if this makes sense.

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • Fixes bug with whitespace on DataTable when merge_duplicate_header=True
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[T4rk1n]

Before adding or modifying a test, I wanted to check if this makes sense.

The bugfix makes sense, screenshot show the desired behavior. Just missing a changelog entry and a test if possible.

[gvwilson]

@leeagustin thanks for the fix - we're happy to merge if you can provide a changelog entry (a test would also be nice)

[gvwilson]

approved pending changelog entry and/or test