Issue963

[laaglu]

Description

A few sentences describing the overall goals of the pull request's commits.

Please check if the PR fulfills these requirements

• [ x] The commit message follows our guidelines: https://github.com/adazzle/react-data-grid/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
On FF only, attempting to resize a column which is also draggable fails (the resizing interaction stop instantaneously and is replaced by a column drag operation)

What is the new behavior?
The new code aligns FF's behavior with other browsers behavior (both interactions, resizing and drag-and-drop are possible and distinct)

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x ] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information: