M3.86: Alt-click the mask thumbnail to view only the mask

transparency.viewOpacityMask {on?, id?} shows only the opacity mask on the
canvas as greyscale coverage (luminance, clip and invert applied; the rest of
the art hidden) and enters mask editing; called again it shows the artwork and
keeps editing. The Transparency panel's mask thumbnail runs it on Alt-click and
is outlined in the accent colour while the mask is viewed. The state is
DocState.mask_view (shown while that mask is edited: DocState::shown_mask) and
reaches the renderer as RenderOptions.mask_view.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/mcp.md

@@ -225,6 +225,11 @@ Groups are not isolated unless Isolate Blending is on: a blend mode inside a gro
 opacity mask, knockout or a clip reaches the art below the group, on screen and in PDF (a knockout group's elements
 then composite against the art below it). SVG has no non-isolated groups, so there such groups stay isolated.
 
+`transparency.viewOpacityMask {on?, id?}` is Alt-clicking the mask thumbnail: the canvas shows only the mask of `id`
+(default: the mask being edited, else the first selected masked object) as greyscale coverage (white = opaque) and the
+mask is edited; called again it shows the artwork, still editing. It is view state of the open document: the canvas
+(and `ui.screenshot`) shows it, exports never do, and it ends when mask editing ends.
+
 ## Clipping masks
 
 `object.clippingMask.make` clips the selected objects by the topmost one, which may be a path, a compound path or a