Let a collection be picked up, rearranged, and emptied after the fact

Collections could be made and filled and never reorganised. Nesting had
a drag; un-nesting had nothing, in either direction — "All photographs"
refused every drop, which is right for a photograph and wrong for a
collection, which has a top level to be returned to. So a collection put
inside another was in there permanently. Right-click deleted an *empty*
collection outright and refused otherwise, which is wrong in both
directions at once: destructive with no confirmation, and no way at all
to delete a collection that held anything without emptying it by hand,
child by child. And a photograph could only leave the collection the
grid was scoped to, since that is the only one a button in the header
can name — the cell's badge says a photograph is in three collections
and never which three.

Three ways in, one vocabulary:

**Hold a row.** The tree is inside a Flickable, which claims any drag
beginning inside it, so with a finger a drag on a row is a scroll until
something says otherwise. The hold is that something. It lifts the row —
drawn before anything moves, so the gesture says it has been understood
— and then what the user does decides which of two things they meant:
move, and it is a rearrangement; let go, and it is the row menu. The
same fork the grid already uses to tell hold-to-select from drag-to-file.
`decide_release` is that fork, and it is tested, because getting it
wrong one way puts a sheet over every tidied tree and the other way
makes the menu unreachable by touch.

**The row menu.** Rename, new collection inside, move to top level,
keep offline, delete. Deleting asks once when there is anything to lose
and says what survives: the photographs stay in the library, and nested
collections move up rather than going with it — which is what the
catalog does, and what a user would never assume. An empty collection
goes on the first press, because a dialogue about losing nothing is how
people learn to dismiss dialogues.

**"Collections…" on a selection.** Every collection the selection is
filed in, each with a count — "3 of 40", so nobody takes forty
photographs out of a collection thirty-seven were never in — and a way
out of any of them without navigating there first.

The long press used to open the offline question by itself. That
question is one item in this menu now: there is one hold per row, and
while it was spent on a single action nothing else the tree can do had
a touch route at all. Nothing is lost — the tray on the row keeps its
tap, and the question gains a full-width control in place of a 30px
icon in a row shorter than the touch minimum.

The row-press handler moves to `collections_ui` with the rest of what a
collection row does; it lived in `library_ui` only because it opened
that prompt.

docs/gestures.md

@@ -5,7 +5,7 @@
 
 Every entry here is extracted from the comment beside the code that implements it, so this file cannot describe a gesture the application does not have. Add one by writing a `GESTURE:` block next to the implementation; there is nowhere else to write it.
 
-30 gestures, in 3 places.
+35 gestures, in 4 places.
 
 ## Develop
 
@@ -25,7 +25,7 @@ Sampling a neutral is the first move of the tonal pass — every colour judgemen
 
 Anchored on the fingers' midpoint, and on the pointer, so the gesture reads as magnifying the picture rather than sliding it about. Double-tap is the way to an exact 1:1; this is the way to everything in between.
 
-<sub>`ui/dr-ui/ui/app.slint:1990`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2064`</sub>
 
 ### Move a magnified photograph about
 
@@ -34,7 +34,7 @@ Anchored on the fingers' midpoint, and on the pointer, so the gesture reads as m
 
 Only once there is something outside the viewport to reach, which is why the cursor becomes a hand exactly then. The view is clamped to the frame: panning past the edge would show undefined area beside the photograph, and that reads as a rendering fault rather than as the end of the picture.
 
-<sub>`ui/dr-ui/ui/app.slint:2143`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2217`</sub>
 
 ### Take back the last change
 
@@ -44,7 +44,7 @@ Only once there is something outside the viewport to reach, which is why the cur
 
 A whole drag is one step, so undo takes back a decision rather than a frame of a gesture. The list is there because arriving six steps back costs what arriving from one does.
 
-<sub>`ui/dr-ui/ui/app.slint:2310`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2384`</sub>
 
 ### Do it again after taking it back
 
@@ -52,7 +52,7 @@ A whole drag is one step, so undo takes back a decision rather than a frame of a
 - **Pointer** — Click it, or press Redo in the History header
 - **Keyboard** — Ctrl+Shift+Z
 
-<sub>`ui/dr-ui/ui/app.slint:2323`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2397`</sub>
 
 ### Copy the settings from this photograph
 
@@ -62,7 +62,7 @@ A whole drag is one step, so undo takes back a decision rather than a frame of a
 
 The panel is the copy that has to work: a tablet has no modifier key to hold and no menu bar to hang the action from. The shortcut is an accelerator for a control that is on screen either way.
 
-<sub>`ui/dr-ui/ui/app.slint:2356`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2430`</sub>
 
 ### Paste the settings onto this photograph
 
@@ -72,7 +72,7 @@ The panel is the copy that has to work: a tablet has no modifier key to hold and
 
 The button names what would be pasted — "3 adjustments", and whether the crop is coming with it — which the shortcut cannot say. Both paste the same scope.
 
-<sub>`ui/dr-ui/ui/app.slint:2368`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2442`</sub>
 
 ### Change which group of adjustments is on screen
 
@@ -82,7 +82,7 @@ The button names what would be pasted — "3 adjustments", and whether the crop
 
 The groups are whatever the operation set declares itself to be about, so there are as many as the pipeline has and no key can be assigned to one of them by name. Stepping is the binding that survives a node being added.
 
-<sub>`ui/dr-ui/ui/app.slint:2396`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2470`</sub>
 
 ### Look at the photograph at 1:1
 
@@ -92,7 +92,7 @@ The groups are whatever the operation set declares itself to be about, so there
 
 Noise reduction and capture sharpening are judgements about single pixels, and a fitted view averages several of the file's into each one on screen — so the frame looks softer than it is and the correction goes too far. The point and the magnification survive opening the next photograph, which is what makes checking the same eye across forty portraits forty keystrokes rather than forty pans.
 
-<sub>`ui/dr-ui/ui/app.slint:2431`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2505`</sub>
 
 ### Move to the next or previous photograph
 
@@ -102,7 +102,7 @@ Noise reduction and capture sharpening are judgements about single pixels, and a
 
 The edit on screen is saved on the way out, so stepping through a folder is as much a departure as going back to the grid and loses nothing.
 
-<sub>`ui/dr-ui/ui/app.slint:2461`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2535`</sub>
 
 ### See the photograph before you edited it
 
@@ -112,7 +112,7 @@ The edit on screen is saved on the way out, so stepping through a folder is as m
 
 Held rather than toggled, and no split screen: a split halves the working image on the tablet the column was sized for, and the comparison photographers describe making is a flick back and forth. It takes no history step, so checking whether a frame is overcooked costs nothing to undo afterwards.
 
-<sub>`ui/dr-ui/ui/app.slint:2585`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2659`</sub>
 
 ### Put one control back to its default
 
@@ -132,6 +132,44 @@ Disabling a layer is the before-and-after a local edit constantly wants, so it i
 
 <sub>`ui/dr-ui/ui/masks.slint:163`</sub>
 
+## Collections sidebar
+
+### Pick a collection up to rearrange the tree
+
+- **Touch** — Press and hold it until it lifts, then drag it
+- **Pointer** — Drag it, or hold it until it lifts and then drag
+
+The tree is inside a Flickable, which claims any drag beginning inside it — so with a finger a drag on a row is a scroll until something says otherwise. The hold is that something, and it is what every mobile list already uses to pick a row up. The row lifts the moment it fires, so the gesture says it has been understood before anything moves.
+
+<sub>`ui/dr-ui/ui/collections.slint:335`</sub>
+
+### Act on a collection — rename, nest, un-nest, delete
+
+- **Touch** — Press and hold the collection, then let go without moving
+- **Pointer** — Right-click it
+
+The hold arms a drag and opens this menu, and which one you get is decided by whether you moved — the same fork the grid uses. One menu for everything done to a row, because there is one hold per row: while the hold opened the offline question by itself, nothing else the tree can do had a touch route.
+
+<sub>`ui/dr-ui/ui/collections.slint:346`</sub>
+
+### Take a collection back out of the one it is nested in
+
+- **Touch** — Hold it, then drag it onto "All photographs" — or let go and choose "Move to top level"
+- **Pointer** — Drag it onto "All photographs", or right-click it and choose "Move to top level"
+
+Nesting is a drag of one row onto another, and its inverse had no gesture at all: "All photographs" refused every drop, which is right for a photograph — it is already in the library — and wrong for a collection, which has a top level to be returned to. Without it a collection dragged into another was in there permanently.
+
+<sub>`ui/dr-ui/ui/collections.slint:356`</sub>
+
+### Rename a collection
+
+- **Touch** — Hold the collection, then "Rename"
+- **Pointer** — Double-click its name, or right-click it and choose "Rename"
+
+Double-click is what a file manager and a Lightroom panel use for the same thing, so it needs no discovering — but nothing on screen says so, which is what the menu item is for.
+
+<sub>`ui/dr-ui/ui/collections.slint:369`</sub>
+
 ## People
 
 ### Pull a face out of the wrong person
@@ -223,7 +261,7 @@ This replaced a double tap, which had no visible state and could take forty phot
 
 "Any of them" is a union and "all of them" is an intersection. The tray is where both terms and the choice between them live, because a filter belongs on the filter bar.
 
-<sub>`ui/dr-ui/ui/library.slint:2096`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2102`</sub>
 
 ### Resize the thumbnails
 
@@ -232,7 +270,7 @@ This replaced a double tap, which had no visible state and could take forty phot
 
 There is no wheel on a tablet, so without the pinch the cell size could only be changed by a control a finger cannot reach.
 
-<sub>`ui/dr-ui/ui/library.slint:2749`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2755`</sub>
 
 ### File photographs in a collection
 
@@ -241,7 +279,7 @@ There is no wheel on a tablet, so without the pinch the cell size could only be
 
 The selection is what the drag carries, which is why selecting several is worth the mode: forty photographs file in one gesture.
 
-<sub>`ui/dr-ui/ui/library.slint:2946`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2952`</sub>
 
 ### Open a photograph
 
@@ -250,7 +288,7 @@ The selection is what the drag carries, which is why selecting several is worth
 
 A tap opens; a tap that *moved* does not. Travel is what separates a deliberate tap from a hand brushing past, and it is the only thing that does: the two are the same length. An earlier version required the finger to dwell 120 ms instead, and that rejected ordinary taps — a real tap is often quicker than a brush.
 
-<sub>`ui/dr-ui/ui/library.slint:3214`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3220`</sub>
 
 ### Rate a photograph without opening it
 
@@ -260,7 +298,7 @@ A tap opens; a tap that *moved* does not. Travel is what separates a deliberate
 
 A star has to take the press without it also reaching the cell, or every rating throws the user into develop.
 
-<sub>`ui/dr-ui/ui/library.slint:3334`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3340`</sub>
 
 ### Choose the frame a folded burst shows
 
@@ -269,7 +307,7 @@ A star has to take the press without it also reaching the cell, or every rating
 
 A folded burst draws its earliest frame, which is a fact about the clock and not a judgement about the photograph — nothing in this application ranks a frame (FR-CULL-5). But the point of a burst is that one of the twelve is better than the other eleven, and the photographer is the only one who knows which. So the choice is offered on the frames themselves, while they are open and side by side, which is the one moment the alternatives are on screen to be compared.
 
-<sub>`ui/dr-ui/ui/library.slint:3465`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3471`</sub>
 
 ### Drop the selection but keep selecting
 
@@ -278,7 +316,7 @@ A folded burst draws its earliest frame, which is a fact about the clock and not
 
 Distinct from Done, which leaves the mode entirely. Clearing keeps it, so the next selection can start straight away.
 
-<sub>`ui/dr-ui/ui/library.slint:4126`</sub>
+<sub>`ui/dr-ui/ui/library.slint:4132`</sub>
 
 ### Select everything the grid is showing
 
@@ -287,4 +325,13 @@ Distinct from Done, which leaves the mode entirely. Clearing keeps it, so the ne
 
 A scoped grid of two hundred frames is two hundred taps otherwise, and "all of them, except those three" is a far more common shape than the taps it took to say it.
 
-<sub>`ui/dr-ui/ui/library.slint:4143`</sub>
+<sub>`ui/dr-ui/ui/library.slint:4149`</sub>
+
+### Take photographs out of a collection
+
+- **Touch** — Select them, then "Collections…" in the selection bar
+- **Pointer** — Select them, then "Collections…" in the selection bar
+
+The badge on a cell says a photograph is filed in three collections and never which. This is the sheet that names them, and the only way out of one the grid is not currently scoped to.
+
+<sub>`ui/dr-ui/ui/library.slint:4264`</sub>