fix: dropdowns should update the month based on their index

[gpbl]

No description provided.

[Copilot]

Pull Request Overview

This PR fixes the dropdown behavior by adjusting the month calculations based on a provided display index.

• Updates the month and year change handlers in DayPicker to use an extra displayIndex parameter
• Introduces a new example and corresponding tests to showcase the dropdown caption layout behavior

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/DayPicker.tsx	Modified handleMonthChange and handleYearChange to incorporate display index offsets for computing the new month
examples/index.ts	Added export for the MultipleMonthsWithDropdown example
examples/MultipleMonthsWithDropdown.tsx	Created a new example demonstrating dropdown usage
examples/MultipleMonthsWithDropdown.test.tsx	Added tests for verifying month and year updates from the dropdown selection

[Copilot]

Subtracting displayIndex from selectedMonth directly can be prone to off-by-one errors if the dropdown indices do not align exactly with the month values. Please add an inline comment explaining the rationale and ensuring that the arithmetic correctly transforms the select input value to the intended month.

Suggested change

	const selectedMonth = Number(e.target.value);
	const selectedMonth = Number(e.target.value);
	// Subtracting `displayIndex` from `selectedMonth` ensures that the selected dropdown value
	// corresponds to the correct month. This assumes that `selectedMonth` is zero-based (0 = January)
	// and `displayIndex` aligns with the dropdown's offset for the displayed months.

[Copilot]

Using displayIndex multiplied by -1 to adjust the month when setting the year may introduce errors if displayIndex is not consistently defined. Consider adding documentation or refactoring to clarify the expected behavior and arithmetic justification.

Suggested change

	month = dateLib.addMonths(month, displayIndex * -1);
	// Adjust the month based on the display index. A negative displayIndex moves the month backward.
	month = dateLib.addMonths(month, -displayIndex);