forgotten to attach the image: 

Deleted: Screenshot 2018-09-10 at 13.54.26.png 115 KB

	Screenshot 2018-09-10 at 13.54.26.png 115 KB View Download

This is due to asynchronous updates of bounds; some transformer may refer to an intermediate bounds.
- this popup is a bubble view, a transient child of the browser window
- ash::SetTransform() in ash/overview/overview_util.h applies the scaling on the browser and its transient descendants
- when the user finishes tab-dragging, Ash resizes the window back to the original size, and then invokes this to scale.

in classic environment, this works as the following flow:
- ash::WindowGrid::AddDraggedWindowIntoOverviewOnDragEnd() invokes TabletModeWindowState::UpdateWindowPosition() to resize the browser window bounds to the original
- the bubble watches the bounds change and resize itself (see views::BubbleDialogDelegateView::OnWidgetBoundsChanged) 
- then ash::SetTransform() to apply the transform to both the browser and the bubble

in Mash / SingleProcessMash:
- WindowGrid invokes the same code. This causes to propagate the new bounds to the browser
- then ash::SetTransform() to apply the transform. The browser window is scaled properly (since its new bounds is known to Ash), yet the bubble still keeps the old bounds.
- the bounds change is propagated to mojo, views::WidgetDialogDelegateView can update to the new position


So Mash invokes ash::SetTransform too earlier than expected -- it should wait for the bounds update of all transient children -- but this happens in browser, it's not known quite well when it ends.