[Request]: share_plus: option to present the iPad share sheet without a popover arrow

Start at the share_plus iOS/macOS UIActivityViewController presentation entry point and inspect how ShareParams.sharePositionOrigin reaches UIPopoverPresentationController. Decide whether to expose permittedArrowDirections, the centered no-arrow option, or both, then add platform-specific coverage and documentation demonstrating the selected behavior.

Plugin

share_plus

Use case

On iPad the share sheet is a popover, and a popover has an arrow pointing at whatever rect was passed as sharePositionOrigin. That is right when the sheet was opened from a visible control. It is wrong whenever there is no such control: a keyboard shortcut, a finished background export, an action whose source has already scrolled away. Any rect passed in those cases is arbitrary, and iPadOS then draws an arrow pointing at a spot the user never touched.

Since 13.3.0 the plugin centres the sheet when no origin is given, which is the correct position — the arrow still points into empty space next to it.

There is no way around this from application code: ShareParams exposes sharePositionOrigin and nothing else that reaches UIPopoverPresentationController, and permittedArrowDirections does not appear anywhere in the iOS plugin. Other share packages on pub.dev wrap the same UIActivityViewController, so they inherit the same behaviour.

This was previously requested in #359 (July 2021), which the stale bot closed in June 2022 without any human reply; that issue is now locked, so I could not comment there. flutter/flutter#74552 asks for the same thing and is still open. Both name the same reference: the export sheet in Procreate, which is centred and arrow-free.

Proposal

Expose the arrow configuration, ignored on every platform other than iOS/macOS. Two parts that can be judged separately:

1. Pass permittedArrowDirections through. Fully documented and mechanical. permittedArrowDirections is settable since iOS 8 and defaults to any; UIPopoverArrowDirection is an OptionSet, so up/down/left/right map one-to-one.

2. An option for "centred, no arrow". This is the part that closes the request, and the implementation deserves a decision rather than a guess, so here is what I measured on an iPad Air 13-inch (M4) simulator, iPadOS 26.5, presenting a UIActivityViewController exactly as the plugin does:

• An empty option set works. With popover.permittedArrowDirections = [] and a 2×2 sourceRect in the centre of the view, the sheet appears centred with no arrow at all, and arrowDirection reads 0 afterwards. It is not documented to work, though — Apple describes unknown only as "The status of the arrow is currently unknown" and says nothing about suppression.
• sourceItem does not help. The docs say that for sourceItem, "In iOS 26 and later, the popover animates from and replaces the specified item" — but that paragraph is about UIBarButtonItem/NSToolbarItem. Anchoring to a plain UIView, which UIPopoverPresentationControllerSourceItem also accepts, still drew an arrow on iPadOS 26.5 (arrowDirection = 2, i.e. down). So the iOS 26 behaviour does not generalise to view items, and Flutter apps have no bar item to anchor to.

I am aware CONTRIBUTING rules out features covering only one platform, and an arrow only exists on iOS and macOS. sharePositionOrigin has exactly the same scope, so there is precedent for a platform-specific parameter here — but I would rather ask than assume. Happy to implement whichever half you consider acceptable.