Fixes single column regression, scrollbar overlap

[ryanml]

Fixes brave/brave-browser#12573, brave/brave-browser#12381

Resolves

Submitter Checklist:

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Test Plan:

Defined in issues

Reviewer Checklist:

• New files have MPL-2.0 license header.
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• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[ryanml]

Natural height when in the "Hide" state was already 41px, but when toggling to "Add", was bumped down to 40.6px, better to just keep this constant.

[emerick]

LGTM

[simonhong]

@ryanml Can you check this again with latest master?
I think this issue seems not reproducible anymore with master now.

[ryanml]

@simonhong looks like neither of these issues are a problem as of #6192, thanks!