Combining merged cells producing incorrect result when used on a non-default cell range

[pcasteran]

Describe the bug
Calling Worksheet.get_values() with combine_merged_cells=True and a range name not starting at column A or row 1 ("B:C" for example) produces incorrect results when expanding the merged cells content.

To Reproduce
Create a worksheet with cells B1 and B2 merged and some other content on C1 and C2.
For example:

A	B	C
	foo	dummy 1
		dummy 2

A call to ws.get_values("B:C", combine_merged_cells=True) outputs:

• ["foo", "dummy 1"]
• ["", "dummy 1"] => no "foo" and "dummy 1" instead of "dummy 2"

Expected behavior
The content of B1 is expanded and available on both rows:

• ["foo", "dummy 1"]
• ["foo", "dummy 2"]

Environment info:

• Operating System: Linux,
• Python version: 3.11
• gspread version: 5.11.3

Additional context
The issue is that the combined_merge_values() function does not take into account the requested cell range to offset the value expansion.

[alifeee]

Hi! Thanks for the report. This is a pretty glaringly wrong problem with combine_merged_cells. Adding this feature (which I did...) has caused a lot of pain, haha.

Perhaps we should have released the feature as experimental. Anyway, I have made a branch with a failing test.

If this test passes, then the bug is fixed. I will attempt the fix when I get time. If you would like to give it a go, please do.