Fix #117572: Top vertical scrollbar tool not working properly

Remove ugly/fragile special case in `view2d_masks()` that would clamp
the scrollbar-masks by the scrubbing UI. It's now possible to pass
custom scrollbar-masks to `View2D` via `UI_view2d_scrollers_draw()`. So
use this instead, making region code control its own masks, rather than
relying on special case handling in generic `View2D` code.

Also update comment in `scroller_activate_init()` to make the implicit
relationship explicit.

Alternative to, and based on the investigation in !117685.

Pull Request: https://projects.blender.org/blender/blender/pulls/117984

source/blender/editors/animation/time_scrub_ui.cc

@@ -183,6 +183,13 @@ void ED_time_scrub_draw(const ARegion *region,
   GPU_matrix_pop_projection();
 }
 
+rcti ED_time_scrub_clamp_scroller_mask(const rcti &scroller_mask)
+{
+  rcti clamped_mask = scroller_mask;
+  clamped_mask.ymax -= UI_TIME_SCRUB_MARGIN_Y;
+  return clamped_mask;
+}
+
 bool ED_time_scrub_event_in_region(const ARegion *region, const wmEvent *event)
 {
   rcti rect = region->winrct;

source/blender/editors/include/ED_time_scrub_ui.hh

@@ -21,6 +21,14 @@ void ED_time_scrub_draw(const ARegion *region,
                         const Scene *scene,
                         bool display_seconds,
                         bool discrete_frames);
+/**
+ * Scroll-bars shouldn't overlap the time scrub UI. So this returns a mask adjusted to exclude it,
+ * which can be passed to #UI_view2d_scrollers_draw().
+ *
+ * \param scroller_mask: Typically #View2D.mask (or something smaller, if further parts have been
+ * masked out already).
+ */
+rcti ED_time_scrub_clamp_scroller_mask(const rcti &scroller_mask);
 
 bool ED_time_scrub_event_in_region(const ARegion *region, const wmEvent *event);
 

source/blender/editors/interface/view2d.cc

@@ -181,13 +181,6 @@ static void view2d_masks(View2D *v2d, const rcti *mask_scroll)
       v2d->vert.xmin = v2d->vert.xmax - scroll_width;
     }
 
-    /* Currently, all regions that have vertical scale handles,
-     * also have the scrubbing area at the top.
-     * So the scroll-bar has to move down a bit. */
-    if (scroll & V2D_SCROLL_VERTICAL_HANDLES) {
-      v2d->vert.ymax -= UI_TIME_SCRUB_MARGIN_Y;
-    }
-
     /* horizontal scroller */
     if (scroll & V2D_SCROLL_BOTTOM) {
       /* on bottom edge of region */

source/blender/editors/interface/view2d_ops.cc

@@ -1908,11 +1908,17 @@ static void scroller_activate_init(bContext *C,
    * - zooming must be allowed on this axis, otherwise, default to pan
    */
   View2DScrollers scrollers;
-  /* Some Editors like the File-browser or Spreadsheet already set up custom masks for scroll-bars
-   * (they don't cover the whole region width or height), these need to be considered, otherwise
-   * coords for `mouse_in_scroller_handle` later are not compatible. */
+  /* Reconstruct the custom scroller mask passed to #UI_view2d_scrollers_draw().
+   *
+   * Some editors like the File Browser, Spreadsheet or scrubbing UI already set up custom masks
+   * for scroll-bars (they don't cover the whole region width or height), these need to be
+   * considered, otherwise coords for `mouse_in_scroller_handle` later are not compatible. This
+   * should be a reliable way to do it. Otherwise the custom scroller mask could also be stored in
+   * #View2D.
+   */
   rcti scroller_mask = v2d->hor;
   BLI_rcti_union(&scroller_mask, &v2d->vert);
+
   view2d_scrollers_calc(v2d, &scroller_mask, &scrollers);
 
   /* Use a union of 'cur' & 'tot' in case the current view is far outside 'tot'. In this cases

source/blender/editors/space_clip/space_clip.cc

@@ -874,7 +874,8 @@ static void graph_region_draw(const bContext *C, ARegion *region)
   ED_time_scrub_draw_current_frame(region, scene, sc->flag & SC_SHOW_SECONDS);
 
   /* scrollers */
-  UI_view2d_scrollers_draw(v2d, nullptr);
+  const rcti scroller_mask = ED_time_scrub_clamp_scroller_mask(v2d->mask);
+  UI_view2d_scrollers_draw(v2d, &scroller_mask);
 
   /* scale indicators */
   {

source/blender/editors/space_graph/space_graph.cc

@@ -333,8 +333,9 @@ static void graph_main_region_draw_overlay(const bContext *C, ARegion *region)
   }
 
   /* scrollers */
+  const rcti scroller_mask = ED_time_scrub_clamp_scroller_mask(v2d->mask);
   /* FIXME: args for scrollers depend on the type of data being shown. */
-  UI_view2d_scrollers_draw(v2d, nullptr);
+  UI_view2d_scrollers_draw(v2d, &scroller_mask);
 
   /* scale numbers */
   {

source/blender/editors/space_sequencer/sequencer_timeline_draw.cc

@@ -1925,5 +1925,6 @@ void draw_timeline_seq_display(const bContext *C, ARegion *region)
 
   const ListBase *seqbase = SEQ_active_seqbase_get(SEQ_editing_get(scene));
   SEQ_timeline_boundbox(scene, seqbase, &v2d->tot);
-  UI_view2d_scrollers_draw(v2d, nullptr);
+  const rcti scroller_mask = ED_time_scrub_clamp_scroller_mask(v2d->mask);
+  UI_view2d_scrollers_draw(v2d, &scroller_mask);
 }