[MDCSelect] Wrong position of opened Select inside Dialog component with option fixed + anchorCorner

[BePo65]

Bug report

When using a MDCSelect with anchorCorner='bottomStart' and fixed=true inside of a MDCDialog, then the selection list (MDCMenu) is positioned too far to the right.

Reason why: MDCMenu has 'position:fixed' (in .mdc-menu-surface--fixed) and mdc-dialog-container has 'transform:scale(1)' (in .mdc-dialog--open .mdc-dialog__container). Therefore mdc-dialog-container creates a stacking context and will act as a containing block for the MDCMenu (instead of the browser window).

Steps to reproduce

1. Create dialog with MDCSelect inside
2. Open dialog
3. Click icon of select to open selection list
4. list is positioned too far to the right

Actual behavior

Left position of menu is calculated based on browser viewport (in MDCMenuSurfaceFoundation.prototype.adjustPositionForHoistedElement_).

Expected behavior

Left position of menu should be calculated based on stacking context.

Your Environment:

Software	Version(s)
MDC Web	5.1.0
Browser	FF 77.0 and Chrome 83.0
Operating System	Windows 10 1909

Additional context

In issue #5590 a hack is proposed (set 'transform:none' in MDCMenu style (but this is only a hack with sideeffects unknown to me).

In issue #2399 the 'solution' was a rework for MDCSelect.

[allan-chen]

This should be fixed by #6012