feat: allow onClickOutside to prevent calendar from closing

[martijnrusschen]

Summary

• Allow onClickOutside callback to prevent the calendar from closing by calling event.preventDefault()
• The callback is now called before the calendar closes, giving users control over the close behavior

Problem

When using custom portal-based dropdowns in the calendar header (e.g., custom month/year selectors), clicking on the portal menu triggers onClickOutside and closes the calendar, even though the click was on a valid UI element.

Previously, the close happened before onClickOutside was called, so users couldn't prevent it.

Solution

Call onClickOutside first, then check event.defaultPrevented before closing:

<DatePicker
  onClickOutside={(event) => {
    // Check if click was inside your portal
    if (myPortalRef.current?.contains(event.target)) {
      event.preventDefault(); // Keeps calendar open
    }
  }}
/>

Test plan

• Added test: "should not close date picker when onClickOutside calls preventDefault"
• All 1472 tests pass
• Lint passes
• Type-check passes

Fixes #5666

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.31%. Comparing base (0231bbd) to head (3700259).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6179   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          30       30           
  Lines        3801     3801           
  Branches     1652     1652           
=======================================
  Hits         3775     3775           
  Misses         25       25           
  Partials        1        1           

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