Two clicks are needed when using the Toolbar icon ("Merge and Center Cells"); the first click un-merges the (partially) merged area – this is the normal use-case when the user wants the merged area to be unmerged – and the second click merges the selected area.

The error message does not appear when using the aforementioned "Merge and Center Cells" toolbar icon.

When using the context menu items ("Merge Cells" and "Unmerge Cells"), or the two equivalent items under the menu "Format > Merge and Unmerge Cells", then the error message will show up.

The two specific items ("Merge Cells" and "Unmerge Cells") are aimed at a range of cells that are either in one status or the other, not mixed. The reason is that, when merging, additional options are available (e.g. when merging, what exactly should happen with the content of each cell in the selected area).

When the additional options are not relevant, and users really want to "expand" the merged area, the simpler and effortless procedure is to select the "new target" range and click once on "Merge and Center Cells", followed by a second one-click on the same toolbar icon.

Changing the current behavior of the two specific actions ("Merge Cells" and "Unmerge Cells") could have negative effects.

First, the potential introduction of bugs in the additional merging options (which is an advantage of Calc over other spreadsheet tools that only support a partial set of options). Since the content of the "other" merged cells is not directly visible by the user, introducing bugs here could lead to loss of data, without users knowing it until it is too late.

The second potential problem is related to automated actions (macros). If an already-existing macro includes either of the "(Un)Merge Cells" actions, changing the result of that same action should be avoided (especially when the user would not know about any potential problem until it is too late).

I think that the only way to safely change the current behavior would be by introducing newly-named actions, producing the "expected behavior" described in comment 0.

Considering the existence of such a simple and direct workaround (i.e. using the "Merge and Center Cells" toolbar icon, which shows no error message as desired, and clicking twice on the same exact place cannot be considered an exceptional effort), I doubt that changing the current behavior of the individual specific "Merge Cells" and "Unmerge Cells" actions is worth the time/resources.

(In reply to ady from comment #1)
> Two clicks are needed when using the Toolbar icon ("Merge and Center
> Cells"); the first click un-merges the (partially) merged area – this is the
> normal use-case when the user wants the merged area to be unmerged – and the
> second click merges the selected area.

But they're not needed. The user wants to merge the two cells - one of them being already merged.

> The error message does not appear when using the aforementioned "Merge and
> Center Cells" toolbar icon.

All the more reason for it not to appear in this case as well :-)

> "Merge Cells"...  aimed at a
> range of cells that are either in one status or the other, not mixed.

Nope. It's a toolbar button, I should be able to press it given any rectangular selection of cells. And - I am able to press it :-)

> The
> reason is that, when merging, additional options are available (e.g. when
> merging, what exactly should happen with the content of each cell in the
> selected area).

That's nice and all, but whether the options are offered on merging or not - it is still possible to merge. If an option isn't offered - a default could be chosen by LO, e.g. the same option chose for the original merge.

> When the additional options are not relevant, and users really want to
> "expand" the merged area, the simpler and effortless procedure is to select
> the "new target" range and click once on "Merge and Center Cells", followed
> by a second one-click on the same toolbar icon.

But I want to click "Merge cells". I don't want to keep that information in the back of my mind and be told "don't do this".

> Changing the current behavior of the two specific actions ("Merge Cells" and
> "Unmerge Cells") could have negative effects.

Nothing that Edit > Undo won't fix.

> Considering the existence of such a simple and direct workaround (i.e. using
> the "Merge and Center Cells" toolbar icon, which shows no error message as
> desired, and clicking twice on the same exact place cannot be considered an
> exceptional effort), I doubt that changing the current behavior of the
> individual specific "Merge Cells" and "Unmerge Cells" actions is worth the
> time/resources.

It's not simple nor direct. I believe you think it is because you are used to it and have accepted that logic. A typical user won't find it simple nor direct: Not simple, because why would "merge cells" fail, but "merge and center" succeed? It's illogical; and not direct, because the user will have no reason to think of trying "merge and center".

Too many (incorrect) assumptions in comment 2.

When I compare:
A: open a context menu and then select one item (i.e. two clicks, on different places on the screen); vs,
B: click twice on a toolbar icon that I already see in front of me,

...I instinctively choose B, even when I am aware of A. I get the same result you are looking for.

There are many things in Calc that don't work as they should, both from a personal subjective opinion POV, and as objective reasoning.

I provided my humble opinion in comment 1, considering the many important things that don't work in Calc (or worse) and that do not have possible alternative procedures. I am not saying that the expected behavior in comment 0 is wrong or unacceptable. But I indeed mentioned potential problems, so any interested developer would take them into account. I am tired of regressions introduced for less-than-essential enhancements.

*** This bug has been marked as a duplicate of bug 143477 ***