Proposal: Add FlyoutPlacementMode.Center to the FlyoutPlacementMode enum.

[AlexanderBlackman]

Desired Behaviour

When FlyoutPlacementMode.Center is selected, the Flyout should centre itself at the centre of the xaml root of the target element.

Rationale

• It is troublesome to use margins to centre the flyout.
• TeachingTipPlacementMode already has "Center", so adding it will improve consistency.

[Lightczx]

Maybe that's why they didn't Add FlyoutPlacementMode.Center at first place

microsoft-ui-xaml/src/dxaml/xcp/dxaml/lib/ContentDialog_Partial.cpp

Lines 1250 to 1272 in 14364e6

	// Here we shamelessly contour ourselves to the Layout system and the whims of
	// the implementers of Popup. The following truths must be known to understand
	// this code:
	// - ContentDialog has two modes, LayouRoot and EntireControl. When it is in the visual
	// tree it cannot add itself to the popup, it's already the child of another UI element.
	// As a result we transplant a specific part of the control template (LayoutRoot) to the
	// popup.
	// When it is not in the visual tree there's no way to apply the control template. We add
	// the entire control to the visual tree in this scenario.
	// - Popup is added to ContentDialog's children when ContentDialog is in LayoutRoot mode to
	// allow inherited properties to work.
	// - When Popup has no parent it positions its contents in the top left of the screen. When it
	// has a parent it positions its contents at the top left of its parent. As a result when we are
	// in LayoutRoot mode we must apply an inverse transform of the top left of the Popup's parent's
	// position.
	// - When a Popup has no parent it does not flip in place for RTL mode. As a result we have to apply
	// a horizontal offset equal to the width of the popup (in our case the screen width) to the Popup
	// when in EntireControl mode.
	// - When a Popup has a parent it determines it position using the top left corner even in RTL mode.
	// as a result we make sure the container is sized to zero pixels.
	// - When a parented Popup is in RTL mode its position is determined by its right edge and not the left edge.
	// - The Arrange pass is busted for RTL mode in Popup so don't expect any autosizing
	// to work correctly. We force feed Popup its Width and Height instead of using Stretch.