Merge pull request #2939 from leeagustin/DataTable-whitespace-fix

Fixes bug with whitespace on DataTable when merge_duplicate_header=True

Author: T4rk1n

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ## Fixed
 
+- [#2939](https://github.com/plotly/dash/pull/2939) Fixes bug with whitespace on DataTable when `merge_duplicate_header=True`. Fixes [#2870](https://github.com/plotly/dash/issues/2870)
 - [#2994](https://github.com/plotly/dash/pull/2994) Keep generated doc-string order for shape or exact props. Fixes [#2990](https://github.com/plotly/dash/issues/2990)
 - [#3011](https://github.com/plotly/dash/pull/3011) Fixed an exception error caused by assigning `None` to array properties with `exact` or `shape` element types. Fixes [#3010](https://github.com/plotly/dash/issues/3010)
 - [#2991](https://github.com/plotly/dash/pull/2991) Add support for URL decoding of the search parameter for pages.

components/dash-table/src/dash-table/components/ControlledTable/index.tsx

@@ -402,7 +402,7 @@ export default class ControlledTable extends PureComponent<ControlledTableProps>
         if (fixed_columns || fixed_rows) {
             const widths = Array.from(
                 r1c1.querySelectorAll(
-                    'table.cell-table > tbody > tr:first-of-type > *'
+                    'table.cell-table > tbody > tr:last-of-type > *'
                 )
             ).map(c => c.getBoundingClientRect().width);