Zoom rungs and viewing aids
Between the canvas tools and the four violet tools, the top panel carries six buttons that change how you look at the photograph and never the photograph itself: three zoom rungs and three overlays. None of them is stored with the frame, none appears in the Changes tab, and none reaches an export.
The whole frame, 100% and 200%
Fit the whole frame draws the whole photograph inside the canvas, with the spare room left empty. 100% fills the canvas edge to edge, so the photograph overflows along one side and the overflow is clipped; how much larger than the whole-frame size depends on the frame's shape and the canvas's: a little on a landscape frame, a good deal on an upright one. 200% draws it twice the size 100% does, for a closer look at a grain or a lash. Press Z to flip between the whole-frame size and 100%. Z, a different rung, or a double-click on the photograph with no tool armed spends any zoom you made with ⌘-scroll or a pinch; clicking the rung that is already lit does not. The canvas covers the rungs and zooming under the pointer.
The rungs make the photograph larger; they do not show the file's own pixels. The cards that open when you rest the pointer on 100% and 200% say one and two screen pixels for every image pixel, and the Sharpening section's guide on the Detail tab says it is judged at 100%. The canvas draws a preview of the photograph no more than 2048 pixels along its long edge, however large the file is, and a rung only scales that preview. To look closer, hold ⌘ and scroll over the photograph, or pinch, up to eight times the size the rung drew; what you see is still an enlarged preview. To judge the file's own pixels, use the zoom on the cull page: Zoom and the focus check.
The focus mask
Press F, or the Focus mask button, to tint the parts of the frame that are actually sharp. It answers the question a fast lens makes you ask on every frame — did the focus land on the eyes or on the ear behind them — with the frame still fitted to the window, instead of by zooming in and hunting.
It is the same sharpness measurement the cull ranks bursts by, taken tile by tile. The bar it tints against is fixed rather than relative to the frame, so a shallow portrait tints its subject plane, a deep landscape tints most of itself, and a frame with nothing sharp in it tints nothing. It is off every time the edit page opens, and it is not re-measured while a slider is under your hand; the mask you already had stays up until the full render replaces it.
The clipping warnings
Press J, or the Clipping warnings button, to tint the pixels with nothing left in them: blown highlights in red, crushed shadows in blue. The histogram at the top of Tone prints how much of the frame has clipped; this draws where, against the same two thresholds, so the overlay and the numbers can never disagree.
Red is any channel at the ceiling, because a single channel clipping is what flattens a red flower long before the frame goes white. Blue is all three channels on the floor, because one channel at zero is ordinary in any saturated shadow. Like the focus mask it is off when the page opens and holds still while you drag.
With the before/after split up, both tints stay on the developed side of the divider only.
Grid and guides
Press G, or the Grid and guides button, to cycle a composition overlay over the whole photograph: off, a thirds grid, a golden ratio grid and a plain centre cross. The View menu has the same cycle as an item that names the overlay you are on, such as View ▸ Grid: Thirds. It is drawn at every zoom. While the crop tool is armed, the crop's own thirds grid over the box is the one guide on screen.
The canvas shade
Press B, or choose View ▸ Canvas: Dark, to cycle the field around the photograph: Dark (Vixlo's own default), Mid grey and Light. The menu item names the shade you are on, so it reads Canvas: Dark, Canvas: Mid grey or Canvas: Light. A mid-grey or light surround changes what your eye judges shadows and colour casts against. Only the field changes; the top panel and the title bar stay as they are.
The grid and the shade are preferences about the app rather than about a frame: both are remembered and restored the next time you open the edit page, on any frame, in any library.