Animations: ping-pong turns on the last key; Fit frames the preview (#112, #111)

Animations.md

@@ -15,7 +15,9 @@ Its two properties are **Frames per second** (default 10) and **Playback**: *Loo
 (ends on the last value and leaves it), *Once (reset)* (ends and puts back what every target held
 before the start) or *Ping-pong*. The last key over all rows decides how long one pass lasts — the
 last key's frame counts as a frame of its own, so keys on 0…9 last one second at 10 fps. A pause at
-the end is a last key that repeats the value. Between two keys the playhead runs on every tick,
+the end is a last key that repeats the value. *Ping-pong* turns on the last key, so neither end
+plays twice: frames keyed on 0…9 play 0…9…1 and start over, eighteen frames a bounce. On the way
+back a value that jumps holds the key the playhead passed last, the one above it. Between two keys the playhead runs on every tick,
 not only on whole frames: at 10 fps a moving `x` moves each frame, the fps only sets how far apart
 the keys are in time.
 
@@ -32,9 +34,9 @@ and property cannot be keyed twice on one animation.
 **The Animation tab.** The header picks the animation, what it **animates** (the root), **+ Row**
 (a dialog: the root or one of its objects, then a property of that; the inspector's
 *Change target…* reopens it to move a row's keys to another property of the same kind), **Play**
-/ **Pause** and **Centre**. Below the header the **preview** shows the root at the playhead, drawn
+/ **Pause** and **Fit**. Below the header the **preview** shows the root at the playhead, drawn
 like the Scene tab draws a scene but never edited — a drag pans, the wheel zooms about the
-pointer, *Centre* frames the content again. It is built from a copy of the root with every row's
+pointer, *Fit* frames the content again. It is built from a copy of the root with every row's
 value at the playhead written over it, so the document stays untouched and undo never hears of
 scrubbing; past the end of a *Once (reset)* pass it shows the starting values, as the player
 would. Under the preview lies the **timeline**: a row per target and property, a frame ruler on
@@ -42,8 +44,7 @@ top. **Double-click** a cell to set a key with the value that holds there (then
 the inspector while the key is selected); click a key to pick it, drag a band to pick several,
 drag keys to move them (one landing on another replaces it), `Delete` removes the picked keys —
 or the whole row when its header is picked. Click or drag the ruler to scrub. The wheel zooms the
-frames about the pointer, `Shift`+wheel scrolls, the middle button (or `Shift`+drag) pans, **Fit**
-returns to frame 0 at the default zoom. Rows are not collapsed or reordered.
+frames about the pointer, `Shift`+wheel scrolls, the middle button (or `Shift`+drag) pans. Rows are not collapsed or reordered.
 
 **Play, stop, pause.** `Play animation` starts an animation on an object (both are ports: the
 object as any action takes it, the animation from an *Animation* literal, a list item tagged