Tell a tap on a photograph from a hand going past

A brush across the grid opened whichever photograph was under it. Travel
was already answered — the Flickable claims the pointer and the press is
cancelled — but a contact that neither travels nor lasts reaches a
TouchArea as an ordinary press and release, and it was landing the user
in develop.

So a finger now has to stay down for `TAP_MIN_MS` before letting go
counts as opening anything. That is the floor under a tap where the 450 ms
`HOLD_DELAY_MS` is the ceiling: below is a graze, between is a tap, above
is a hold that starts a selection. One scale, three gestures.

Only a finger is held to it. A mouse click is a discrete decision made by
a button and is routinely over in thirty milliseconds, so `cell-pressed`
now reports whether a finger did it — the same finger-id convention the
pinch arbitration beside it already uses — and the dwell applies to touch
alone.

A graze still *selects* the cell it landed on, because the press already
did that. That is the right failure mode: something visible and
reversible rather than a silent nothing, and rather than develop.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/collections_ui.rs

@@ -157,6 +157,16 @@ pub struct CollectionsController {
     /// develop view on the second ctrl-click. Slint does not report modifiers on
     /// `clicked`, so the press records them and the click consults this.
     modified_press: std::cell::Cell<bool>,
+    /// TRACES: FR-UI-4
+    /// When the press that began the current gesture landed, and whether a
+    /// finger did it.
+    ///
+    /// Read by the click that follows, to tell a tap from a graze — see
+    /// [`CollectionsController::press_was_a_graze`]. `None` between gestures,
+    /// which a click with no press before it is treated as: it cannot have been
+    /// deliberate if nothing pressed.
+    pressed_at: std::cell::Cell<Option<std::time::Instant>>,
+    touch_press: std::cell::Cell<bool>,
     /// TRACES: FR-UI-2 | FR-UI-4
     /// Whether a tap in the grid selects rather than opens.
     ///
@@ -333,6 +343,38 @@ impl CollectionsController {
         self.modified_press.get()
     }
 
+    /// TRACES: FR-UI-4
+    /// Whether the contact that is ending was too brief to have meant anything.
+    ///
+    /// A hand crossing a tablet on its way to the scroll it intended produces a
+    /// press and a release a few tens of milliseconds apart, in the same place —
+    /// indistinguishable, to a `TouchArea`, from a tap, and it was opening
+    /// whichever photograph happened to be under the knuckle. Travel is already
+    /// answered (the Flickable claims the pointer and the press is cancelled);
+    /// what was left is the contact that does not travel and does not last.
+    ///
+    /// **Only a finger is held to this.** A mouse click is a discrete decision
+    /// made by a button and is routinely over in thirty milliseconds; applying
+    /// a dwell to it would make the desktop feel broken to fix a problem the
+    /// desktop does not have.
+    ///
+    /// And it only withholds the *open*. The press has already selected the
+    /// cell under the finger, which is the right failure mode: a graze leaves
+    /// something visible and reversible on screen rather than silently doing
+    /// nothing, and rather than throwing the user into develop.
+    pub fn press_was_a_graze(&self) -> bool {
+        if !self.touch_press.get() {
+            return false;
+        }
+        match self.pressed_at.get() {
+            Some(at) => at.elapsed() < std::time::Duration::from_millis(TAP_MIN_MS),
+            // A click with no press recorded before it. Not a graze — there is
+            // nothing to say it was one — and refusing it would be the one way
+            // this rule could make a photograph unopenable.
+            None => false,
+        }
+    }
+
     /// Selected image ids, in a stable order.
     pub fn selected(&self) -> Vec<ImageId> {
         self.selection.borrow().iter().copied().collect()
@@ -1032,6 +1074,22 @@ const SPRING_DELAY_MS: u64 = 500;
 /// the hand lets go first, having concluded nothing was going to happen.
 pub(crate) const HOLD_DELAY_MS: u64 = 450;
 
+/// TRACES: FR-UI-4
+/// How long a finger must stay down before letting go counts as opening a
+/// photograph.
+///
+/// **The floor under a tap, where `HOLD_DELAY_MS` is the ceiling.** Between the
+/// two is a tap; below is a graze that only selects; above is a hold that
+/// starts a selection. The three have to be one scale or the gesture set stops
+/// being learnable.
+///
+/// 120 ms is a tenth of the hold and about twice a brush. It costs nothing in
+/// felt latency because it does not *delay* anything — the open still happens
+/// on release, and this only decides whether that release counted — so the
+/// error it can make is one-sided: an unusually quick deliberate tap selects
+/// instead of opening, and the photograph is one further tap away.
+pub(crate) const TAP_MIN_MS: u64 = 120;
+
 /// TRACES: FR-UI-2 | FR-UI-4
 /// Start the timer that turns a held cell into a selection.
 ///
@@ -1501,13 +1559,17 @@ pub fn wire<S, R, P, C>(
         let weak = window.as_weak();
         let ctl = ctl.clone();
         let visible = visible_ids.clone();
-        window.on_library_cell_pressed(move |row, ctrl_held, shift_held| {
+        window.on_library_cell_pressed(move |row, ctrl_held, shift_held, touch| {
             let Some(w) = weak.upgrade() else { return };
             let ids = visible();
 
             // Consulted by the click that follows: a modified press is building
             // a selection and must not also navigate to develop.
             ctl.modified_press.set(ctrl_held || shift_held);
+            // And so is this: how long the contact lasts is what separates a
+            // tap from a graze, and only the press knows when it started.
+            ctl.pressed_at.set(Some(std::time::Instant::now()));
+            ctl.touch_press.set(touch);
 
             // Where the window starts, so the press is recorded as the ordinal
             // it is rather than as a row that stops meaning this photograph on
@@ -2739,6 +2801,52 @@ mod tests {
         Vec::new()
     }
 
+    /// TRACES: FR-UI-4
+    /// A hand brushing the tablet on its way to a scroll must not open a
+    /// photograph. It still *selects* one — the press did that, and something
+    /// visible and reversible is the right thing to be left with.
+    #[test]
+    fn a_graze_does_not_open_a_photograph() {
+        let ctl = CollectionsController::new(crate::activity::ActivityLog::new());
+        ctl.touch_press.set(true);
+        ctl.pressed_at.set(Some(std::time::Instant::now()));
+        assert!(
+            ctl.press_was_a_graze(),
+            "a release in the same instant as the press is not a tap"
+        );
+    }
+
+    /// And a finger that stayed down is a tap, not a graze.
+    #[test]
+    fn a_deliberate_tap_opens_a_photograph() {
+        let ctl = CollectionsController::new(crate::activity::ActivityLog::new());
+        ctl.touch_press.set(true);
+        ctl.pressed_at.set(Some(
+            std::time::Instant::now() - std::time::Duration::from_millis(TAP_MIN_MS + 10),
+        ));
+        assert!(!ctl.press_was_a_graze());
+    }
+
+    /// The desktop is not held to the dwell. A mouse button is a discrete
+    /// decision and is routinely down for thirty milliseconds.
+    #[test]
+    fn a_mouse_click_is_never_a_graze() {
+        let ctl = CollectionsController::new(crate::activity::ActivityLog::new());
+        ctl.touch_press.set(false);
+        ctl.pressed_at.set(Some(std::time::Instant::now()));
+        assert!(!ctl.press_was_a_graze());
+    }
+
+    /// The floor has to sit under the ceiling, or there is no tap between them:
+    /// every press would be either a graze or a hold.
+    #[test]
+    fn a_tap_has_room_between_a_graze_and_a_hold() {
+        // A `const` block, so this is a compile error rather than a test
+        // failure: the two numbers are constants, and a gesture set with no
+        // room for a tap in it should not get as far as being run.
+        const { assert!(TAP_MIN_MS < HOLD_DELAY_MS) };
+    }
+
     #[test]
     fn dragging_a_collection_onto_another_moves_it() {
         // The gesture the tree rearrangement exists for: no images carried, a

ui/dr-ui/src/library_ui.rs

@@ -4411,6 +4411,13 @@ pub fn wire<F>(
             if coll_for_click.press_was_modified() {
                 return;
             }
+            // TRACES: FR-UI-4
+            // And a graze is not a tap. The press has already selected the cell,
+            // so the user sees what they touched; what they do not get is a
+            // photograph opened by a hand on its way past.
+            if coll_for_click.press_was_a_graze() {
+                return;
+            }
             let path = ctl.paths.borrow().get(i as usize).cloned();
             if let Some(path) = path {
                 // Leave the grid for the develop view. The status bar's