Fix touchpad pan gesture suppressed when editing masks

A two-finger scroll (no CTRL) fell through to zoom whenever a mask
overlay was visible, even with the pointer off the mask. gesture_pan
now declines the pan only when the scroll would actually be consumed
by the mask (pointer over a shape, or creating a size-adjustable
shape) via the new dt_masks_scroll_over_mask() predicate. Elsewhere
on the canvas, and while drawing a path (which has no scroll-adjustable
parameter during creation), the scroll pans the image as expected.

Author: da-phil

src/develop/masks.h

@@ -603,6 +603,10 @@ gboolean dt_masks_events_mouse_scrolled(struct dt_iop_module_t *module,
                                         const float y,
                                         const gboolean up,
                                         const uint32_t state);
+// Return TRUE if scrolling over the center view should adjust the visible
+// mask (size/border/opacity) instead of zoom/pan. Returns FALSE while drawing
+// a path, since path creation has no scroll-adjustable parameter.
+gboolean dt_masks_scroll_over_mask(void);
 void dt_masks_events_post_expose(const struct dt_iop_module_t *module,
                                  cairo_t *cr,
                                  const int32_t width,

src/develop/masks/masks.c

@@ -1225,12 +1225,27 @@ gboolean dt_masks_events_button_pressed(dt_iop_module_t *module,
   if(form->functions)
     return form->functions->button_pressed(module, pzx, pzy, pressure,
                                            which, type, state, form, 0, gui, 0)
-      || which == 3; // swallow right-clicks even when not handled so
-                     // right-drag rotate is disabled when forms
-                     // visible
+      || which == 3; // swallow right-clicks so right-drag rotate is disabled
   return FALSE;
 }
 
+gboolean dt_masks_scroll_over_mask(void)
+{
+  const dt_masks_form_t *form = darktable.develop->form_visible;
+  const dt_masks_form_gui_t *gui = darktable.develop->form_gui;
+  if(!form || !gui)
+    return FALSE;
+
+  // During shape creation, scroll adjusts size/border and stays over the mask.
+  // Paths/polygons are built node by node and do not use scroll here.
+  if(gui->creation)
+    return !(form->type & DT_MASKS_PATH);
+
+  // Otherwise form_visible is the group. group_edited is the active sub-form or
+  // -1 when none. Scroll should count only when a sub-form is active.
+  return gui->group_edited >= 0;
+}
+
 gboolean dt_masks_events_mouse_scrolled(dt_iop_module_t *module,
                                         const float pzx,
                                         const float pzy,
@@ -1250,8 +1265,7 @@ gboolean dt_masks_events_mouse_scrolled(dt_iop_module_t *module,
 
   if(gui)
   {
-    // for brush, the opacity is the density of the masks, do not
-    // update opacity here for the brush.
+    // Do not update brush opacity here; it is mask density.
     if(gui->creation && dt_modifier_is(state, GDK_CONTROL_MASK))
     {
       float opacity = dt_conf_get_float("plugins/darkroom/masks/opacity");

src/views/darkroom.c

@@ -4217,12 +4217,14 @@ gboolean gesture_pan(dt_view_t *self,
   (void)state;
   if(!dev) return FALSE;
 
-  // Mask editing (brush etc.) uses scroll for tool parameters.
+  // If pointer is over an active mask, let scroll go to the mask handler;
+  // otherwise allow two-finger scroll to pan the image.
   if(dev->form_visible
-     && !darktable.develop->darkroom_skip_mouse_events)
+     && !darktable.develop->darkroom_skip_mouse_events
+     && dt_masks_scroll_over_mask())
   {
     dt_print(DT_DEBUG_INPUT,
-             "[darkroom pan] ignored: mask form active");
+             "[darkroom pan] ignored: pointer over active mask");
     return FALSE;
   }