Popover: assess all `position` to `placement` conversions, add unit tests

[ciampo]

With the refactor of Popover to using floating-ui internally (#40740), a new placement prop was introduced with the objective of replacing the legacy position prop.

Currently, there is a function converting position to placement, but:

• its logic is complicated and difficult to follow
• its logic could potentially miss some edge cases:
  • it looks like position accepted the undocumented format [yAxis] [xAxis] [corner])
  • it looks like the new placement prop doesn't allow the Popover to be placed centered on top of its anchor
• its logic is not covered by unit tests (which makes the previous checks necessary)

Therefore, to improve the situation and get ready to deprecate the position prop, we should:

1. Assess that all possible values of position are converted correctly (or in a best-effort way) to the corresponding placement
2. Write unit tests
3. Potentially make changes to the conversion logic to respect the tests added from the previous point

[ciampo]

I went ahead and checked out the repository before #40740 was merged. I was therefore able to render a Popover for all possible values of the position prop (even the ones that don't really make much sense), simulating a Popover wider and taller than its anchor.

I then repeated the same thing on trunk, taking a screenshot of all possible placement values.

Here's the comparison:

All position values

Popover wider than anchor	Anchor wider than Popover

All placement values

Popover wider than anchor	Anchor wider than Popover

[ciampo]

Based on the images above, this should be the expected conversion:

(disclaimer: for completeness, I listed all possible combinations of values for the position prop, even if some of them don't really make sense)

Position	Placement	Notes
middle	top	There is no equivalent placement in floating-ui, use top as fallback
bottom	bottom	-
top	top	-
middle left	left	-
middle center	top	There is no equivalent placement in floating-ui, use top as fallback
middle right	right	-
bottom left	bottom-end	Not a perfect match, but likely the closest
bottom center	bottom	-
bottom right	bottom-start	Not a perfect match, but likely the closest
top left	top-end	Not a perfect match, but likely the closest
top center	top	-
top right	top-start	Not a perfect match, but likely the closest
middle left left	left	-
middle left right	left	Not a perfect match, but likely the closest (not sure if this position even makes sense)
middle left bottom	left-end	Not a perfect match, but likely the closest (this one is really up to interpretation)
middle left top	left-start	Not a perfect match, but likely the closest (this one is really up to interpretation)
middle center left	top	There is no equivalent placement in floating-ui, use top as fallback (not sure if this position even makes sense)
middle center right	top	There is no equivalent placement in floating-ui, use top as fallback (not sure if this position even makes sense)
middle center bottom	top	There is no equivalent placement in floating-ui, use top as fallback (not sure if this position even makes sense)
middle center top	top	There is no equivalent placement in floating-ui, use top as fallback (not sure if this position even makes sense)
middle right left	right	Not a perfect match, but likely the closest (not sure if this position even makes sense)
middle right right	right	-
middle right bottom	right-end	Not a perfect match, but likely the closest
middle right top	right-start	Not a perfect match, but likely the closest
bottom left left	bottom-end	Not a perfect match, but likely the closest
bottom left right	bottom-end	-
bottom left bottom	bottom-end	Not a perfect match, but likely the closest (not sure if this position even makes sense)
bottom left top	bottom-end	Not a perfect match, but likely the closest (not sure if this position even makes sense)
bottom center left	bottom	not sure if this position even makes sense
bottom center right	bottom	not sure if this position even makes sense
bottom center bottom	bottom	not sure if this position even makes sense
bottom center top	bottom	not sure if this position even makes sense
bottom right left	bottom-start	-
bottom right right	bottom-start	Not a perfect match, but likely the closest
bottom right bottom	bottom-start	Not a perfect match, but likely the closest (not sure if this position even makes sense)
bottom right top	bottom-start	Not a perfect match, but likely the closest (not sure if this position even makes sense)
top left left	top-end	Not a perfect match, but likely the closest
top left right	top-end	-
top left bottom	top-end	Not a perfect match, but likely the closest (not sure if this position even makes sense)
top left top	top-end	Not a perfect match, but likely the closest (not sure if this position even makes sense)
top center left	top	not sure if this position even makes sense
top center right	top	not sure if this position even makes sense
top center bottom	top	not sure if this position even makes sense
top center top	top	not sure if this position even makes sense
top right left	top-start	-
top right right	top-start	Not a perfect match, but likely the closest
top right bottom	top-start	Not a perfect match, but likely the closest (not sure if this position even makes sense)
top right top	top-start	Not a perfect match, but likely the closest (not sure if this position even makes sense)

[ciampo]

In #44377 I introduced a comprehensive set of unit tests to assess the current status of the returned placement values for every possible position.

*: bottom placements marked with an asterisks are fallback values used when position has middle center values, since there's not equivalent placement value

position	placement expected by test specs	placement received (as on trunk)	matches expected value?
'middle'	'bottom'*	undefined	❌
'bottom'	'bottom'	'bottom'	✅
'top'	'top'	'top'	✅
'middle left'	'left'	'left'	✅
'middle center'	'bottom'*	'center'	❌
'middle right'	'right'	'right'	✅
'bottom left'	'bottom-end'	'bottom-end'	✅
'bottom center'	'bottom'	'bottom'	✅
'bottom right'	'bottom-start'	'bottom-start'	✅
'top left'	'top-end'	'top-end'	✅
'top center'	'top'	'top'	✅
'top right'	'top-start'	'top-start'	✅
'middle left left'	'left'	'left'	✅
'middle left right'	'left'	'left'	✅
'middle left bottom'	'left-end'	'left'	❌
'middle left top'	'left-start'	'left'	❌
'middle center left'	'bottom'*	'center'	❌
'middle center right'	'bottom'*	'center'	❌
'middle center bottom'	'bottom'*	'center'	❌
'middle center top'	'bottom'*	'center'	❌
'middle right left'	'right'	'right'	✅
'middle right right'	'right'	'right'	✅
'middle right bottom'	'right-end'	'right'	❌
'middle right top'	'right-start'	'right'	❌
'bottom left left'	'bottom-end'	'bottom-start'	❌
'bottom left right'	'bottom-end'	'bottom-end'	✅
'bottom left bottom'	'bottom-end'	'bottom-end'	✅
'bottom left top'	'bottom-end'	'bottom-end'	✅
'bottom center left'	'bottom'	'bottom-start'	❌
'bottom center right'	'bottom'	'bottom-end'	❌
'bottom center bottom'	'bottom'	'bottom'	✅
'bottom center top'	'bottom'	'bottom'	✅
'bottom right left'	'bottom-start'	'bottom-start'	✅
'bottom right right'	'bottom-start'	'bottom-start'	✅
'bottom right bottom'	'bottom-start'	'bottom-start'	✅
'bottom right top'	'bottom-start'	'bottom-start'	✅
'top left left'	'top-end'	'top-start'	❌
'top left right'	'top-end'	'top-end'	✅
'top left bottom'	'top-end'	'top-end'	✅
'top left top'	'top-end'	'top-end'	✅
'top center left'	'top'	'top-start'	❌
'top center right'	'top'	'top-end'	❌
'top center bottom'	'top'	'top'	✅
'top center top'	'top'	'top'	✅
'top right left'	'top-start'	'top-start'	✅
'top right right'	'top-start'	'top-start'	✅
'top right bottom'	'top-start'	'top-start'	✅
'top right top'	'top-start'	'top-start'	✅

The inconsistencies can be split into four main groups:

1. 'middle' and 'middle center' positions: these values are not handles correctly by the positionToPlacement function, since the converted placement is either undefined or center (which is not even a valid placement). Since floating-ui doesn't have a corresponding placement for the middle center positions, my suggestion is to use a fallback value of bottom (which is also floating-ui's default value).
2. 'middle [left|right] [bottom|top]' positions: for these positions, positionToPlacement currently returns vertically centered [left|right] placement, while the screenshots above suggest that the resulting placement should be aligned to the top or the bottom (i.e. '-start' or an '-end' alignment).
3. '[bottom|top] center [left|right]' positions: for these positions, positionToPlacement currently returns an aligned -[start|end] placement, while the screenshots above suggest that the resulting placement should be centered on the x axis (ie. only 'bottom' or 'top' values)
4. '[bottom|top] left left' positions: for these positions, positionToPlacement currently returns a '-start' alignment, while the screenshots above suggest more of an '-end' alignment. An '-end' alignment would also be more consistent with how other positions are converted (e.g top left is converted to an -end alignment and the popover looks like it's positioned in the same way, top right right is converted to a -start alignment...)