Design Choices
Fuji is a handful of small decisions, each made with intent, and each a departure from what the other programs on your machine do. None of them is a feature with a button. They are the places where Fuji behaves differently from every other viewer you have used, and the difference is the design. This page names each one and makes the case for it. The keys and the drags are on the Diamond Table page.
Viewport Persistence
A viewer that fits each picture to the window as it arrives throws away your zoom and your pan on every flip. You find the detail, you zoom in on it, you press the arrow key, and the next picture arrives fitted to the window again, with the detail back at its original size somewhere in the frame. Fuji's light table holds one number, the size of the diamond every picture is sized to, and one place, where you have panned to, and neither changes when you flip. Fuji calls this Viewport Persistence.
Set a camera on a tripod and take fifty frames of the same scene, and this is what you want: pan to the figure, zoom in, and flip. The frame stands still while the figure moves and the light changes. It holds through a burst, through a time lapse, through a folder of scans of the same page, through a set of screenshots of the same window, through anything where the same part of the frame matters from one picture to the next. And it holds when the pictures do not match. Mix a portrait into a folder of landscapes, or a phone screenshot into a folder of photographs, and the viewport is not lost, because the diamond fits every shape the same way, centered on the same point, and the next landscape finds you exactly where you were.
Pan anywhere
Most viewers stop the picture at the edge of the window, as if the frame were a window you were looking through and the picture a wall behind it. Drag the corner toward the middle and it stops at the edge of the screen, and the corner stays in the corner. The corner is often the part. A signature, a watermark, the date a camera burned into the frame, the border of a scan, the caption under a comic panel, the status bar of a screenshot: the things a collection carries at its edges are the things you examine, and a viewer that keeps them at the edge of the screen keeps them where you look last.
Fuji's plane has no edge. Mouse drag, and the picture moves under your hand, and keeps moving: past the edge, off the screen, until the corner sits in the middle of your display if that is where you want it. Hold the right button and drag up, and the picture zooms about the point you grabbed, so a detail in a corner grows toward you rather than sliding away. When you have seen it, Space puts the picture back in the center, sized to the diamond.
Who opens what
Making a program your picture viewer has had one screen for as long as you have been installing them: a list of every extension the program knows, a check box in every row, and Select All at the top. Microsoft drew that screen itself in Windows Vista, a table it called Set associations for a program, and its documentation for Default Programs told programs to use it rather than draw their own. The same page recommends a dialog at first run asking to be the default, and adds, "Always include a check box in this UI that is selected by default and that presents the option not to be asked again." You have ticked that box. Every viewer that asked you to tick it lost the argument the moment it asked.
Fuji shows you who opens what. Its settings hold a chip for every kind of file Fuji opens, and the chips are grouped under the program that opens that kind now: Opens with Fuji first, then Currently opens with 'Paint', Currently opens with 'Photos', and so on, the program holding the most kinds first, and Currently opens with nothing last. The chips wrap in rows like tags, so a long list of formats is a paragraph you read at a glance, not a table you scroll. You see that one program holds your BMPs and PNGs, another holds your JPEGs, and nothing at all holds your WebPs, which a table of rows shows you one row at a time, if it shows you at all. Rest the pointer on a chip and a card says what the format is and where it came from, and offers one button, Choose Fuji. On the Mac that click is the whole transaction, done at once with no dialog. On Windows, where only Windows' own screen can finish a change since Windows 8, the chip turns a warning color and a link takes you to that screen and back.
Fuji never asks at launch, never takes a type when it is installed, and never takes one back without a second click. Installing or running Fuji is not permission. The Default Programs page says that too, in one sentence Fuji agrees with: "An application should never reclaim a default without asking the user."
Never a HUD
You know this one. You double-click a video, and a bar of controls appears on top of it. You wait, and the bar fades. You move the mouse to see how long the clip is, and the bar comes back, over the part of the picture you were looking at. You pause, you come back with a snack, you press play, and there it is again, fading over the first seconds you wanted to see. You have been opening video files on a desktop computer for thirty years, and for most of them this is how it has gone. QuickTime Player's controls appear when the pointer moves over the video, and Apple's guide to QuickTime Player tells you that "you can drag the controls to move them out of the way", which concedes where they are. VLC shows a controller over a full-screen video until you find the preference that turns it off, and hides it again after the same timeout that hides the mouse. The rest follow.
The fading overlay was designed for the feature film, and it is right for one. The movie runs two hours. Its first minutes are credits over a song, and nothing is lost while the controls fade; pause for a snack, press play, and the bar fades over credits that are still rolling. The idea is older than the desktop. A television of the late 1970s drew its channel number and the time over the picture because the screen was the only display it had, and the VCR's on-screen programming followed, so a few seconds of overlay on a two-hour program was the price of any display at all; the history of the on-screen display is on Wikipedia. That overlay is the HUD, the head-up display, a name borrowed from the cockpit for anything drawn over the picture.
QuickTime did not start there. QuickTime 1.0, in 1991, put its controller in a bar of its own under the picture: a movie's window was the movie's natural size plus the height of the control bar, in Apple's documentation, and the picture was under nothing. QuickTime X, in 2009, moved the controls over the picture and faded them, and Macworld found that a small movie could no longer open at its own size, "to accommodate its enormous floating controller bar". The overlay has been the shape of every player since.
But a feature film is not the video on your drive, and it has not been for a long time. The clips in a collection are short: seconds off a CD-ROM, a loop saved from a forum, a TikTok. TikTok began at fifteen seconds, allowed a minute, and did not reach three until July 2021. The clip is over before the overlay has faded, and while it was there it covered the one part you opened the clip to see. A short video is a picture that moves, and a picture with a bar across it is not whole.
So Fuji has a rule, Never a HUD: nothing is ever drawn over a video. Not a control bar, not a title, not a progress line, not a volume readout, and nothing that appears because the mouse moved. The video shows whole, from its first frame to its last, the way Fuji shows every picture. What Fuji has to say about a video sits beside it, and the keyboard does what a hover used to do.