[Spreadsheet] Custom cell editor combobox does not show and is not active on merged cells

[Shufflepants]

Description

After 8317 it seems there are still issues with merged cells that are causing an issue. When a ComboBox editor is on a merged cell, something about the rendering process causes the ComboBox to not be visible/active. There is even a split second while the page is loading where the highlighted background of a properly visible ComboBox appears in the first cell of a set of merged cells before the merging happens and the ComboBox disappears.
You can see that cell B3 should have a visible ComboBox like B18, but it does not.

After a single click on the cell, the combobox becomes visible, but does not open.

It is as though the setShowCustomEditorOnFocus is set to true even though it is not. And ComboBox custom editors that are not in merged cells properly show up and only require 1 click to open the drop-down.

Expected outcome

The drop-down arrow of a ComboBox custom editor in a spreadsheet should be visible upon initial load, and there should only be a single click required to open the drop down and have the text field selected.

Minimal reproducible example

Using the provided example project scroll-with-editors-formatting-bug.zip that was used for 8317, simply change the 4 on line 68 to a 2 so that one of the ComboBoxs is now in the same cell as one of the merged cells.

Steps to reproduce

You can reproduce this by putting a ComboBox editor in the same cell as an existing merged cell. There are two ComboBoxs in the screenshots, one on B3 and one on B18, but only the one on B18 shows up without needing to be clicked on first.

Environment

Vaadin version(s): 24.9.8
OS: Windows 11

Browsers

Firefox