Description:
After a recent commit (e27d4cc31e04be4c47b5085dfa2363ee45457e8a) that changed the way cell data is managed using an item pool (using unorderd_set instead in the latest source code), there appear to be issues with the rendering of borders in merged cells. The visual appearance of the spreadsheet is incorrectly altered when exporting to PDF, as demonstrated by the differences between the "test_correct.pdf" and "test_wrong.pdf" files provided in the attached Google Drive link: https://drive.google.com/drive/folders/1Ko6gLg7fNzzCwjmTHoenB01SnyXG_5c9?usp=sharing.

Steps to Reproduce:
1. Download the test spreadsheet file "test.xlsx" from the following Google Drive link:
https://drive.google.com/drive/folders/1Ko6gLg7fNzzCwjmTHoenB01SnyXG_5c9?usp=sharing

2. Check out the LibreOffice code from the repository at a commit before the problematic changes were introduced, specifically before commit e27d4cc31e04be4c47b5085dfa2363ee45457e8a.

3. Build and run LibreOffice from this pre-commit code.

4. Open the "test.xlsx" file in LibreOffice Calc and export it as a PDF. The resulting PDF should look identical to the "test_correct.pdf" file provided in the Google Drive link.

5. Now, check out the LibreOffice code at a commit after the problematic changes were introduced, specifically after commit e27d4cc31e04be4c47b5085dfa2363ee45457e8a.

6. Build and run LibreOffice from this post-commit code.

7. Open the same "test.xlsx" file in LibreOffice Calc and export it as a PDF. The resulting PDF should now have visual differences compared to the "test_correct.pdf" file. The exported PDF should look like "test_wrong.pdf" file provided in the Google Drive link, where the border of merged cells is not drawn correctly.

Actual Results:
The exported PDF is "test_wrong.pdf" file provided in the Google Drive link: https://drive.google.com/drive/folders/1Ko6gLg7fNzzCwjmTHoenB01SnyXG_5c9?usp=sharing

Expected Results:
The exported PDF should be "test_correct.pdf" file provided in the Google Drive link: https://drive.google.com/drive/folders/1Ko6gLg7fNzzCwjmTHoenB01SnyXG_5c9?usp=sharing


Reproducible: Always


User Profile Reset: No

Additional Info:
During debugging, I focused on the lclSetMergedRange function, which appears to correctly recognize the range of merged cells. The problem does not seem to lie within this function.

However, I suspect that the root cause of the bug is related to the code changes introduced to optimize memory usage. Specifically, the modifications made to manage cell data using an item pool (using unorderd_set instead in the latest source code), including the introduction of the PUTCELL macro and the changes in how cell borders are stored and retrieved, are likely responsible for the incorrect rendering of borders in merged cells.

Further investigation and debugging efforts will be focused on these memory optimization changes to identify the precise cause of the issue and develop a suitable fix.