UI: Auto-scroll to keep active text buttons in view

If a text button is activated that is not in view (i.e. scrolled away),
the scrolling will now be adjusted to have it in view (with some
small additional margin). While entering text, the view may also be
updated should the button move out of view, for whatever reason. For the
most part, this feature shouldn't be needed and won't kick in, except
when a clicked on text button is partially out of view or very close to
the region edge. It's however quite important for the previously
committed feature, that is, pressing Ctrl+F to start searching in a UI
list. The end of the list where the scroll button appears may not be in
view. Plus while filtering the number of visible items changes so the
scrolling has to be updated to keep the search button visible.

Note that I disabled the auto-scrolling for when the text button spawned
an additional popup, like for search-box buttons. That is because
current code assumes the button to have a fixed position while the popup
is open. There is no code to update the popup position together with the
button/scrolling.

I also think that the logic added here could be used in more places,
e.g. for the "ensure file in view" logic the File Browser does.

source/blender/editors/interface/interface_utils.c

@@ -29,6 +29,8 @@
 #include "DNA_object_types.h"
 #include "DNA_screen_types.h"
 
+#include "ED_screen.h"
+
 #include "BLI_alloca.h"
 #include "BLI_listbase.h"
 #include "BLI_math.h"
@@ -38,6 +40,7 @@
 
 #include "BLT_translation.h"
 
+#include "BKE_context.h"
 #include "BKE_lib_id.h"
 #include "BKE_report.h"
 
@@ -48,6 +51,7 @@
 #include "UI_interface.h"
 #include "UI_interface_icons.h"
 #include "UI_resources.h"
+#include "UI_view2d.h"
 
 #include "WM_api.h"
 #include "WM_types.h"
@@ -773,6 +777,98 @@ bool UI_but_online_manual_id_from_active(const struct bContext *C, char *r_str,
   return false;
 }
 
+/* -------------------------------------------------------------------- */
+
+static rctf ui_but_rect_to_view(const uiBut *but, const ARegion *region, const View2D *v2d)
+{
+  rctf region_rect;
+  ui_block_to_region_rctf(region, but->block, &region_rect, &but->rect);
+
+  rctf view_rect;
+  UI_view2d_region_to_view_rctf(v2d, &region_rect, &view_rect);
+
+  return view_rect;
+}
+
+/**
+ * To get a margin (typically wanted), add the margin to \a rect directly.
+ *
+ * Based on #file_ensure_inside_viewbounds(), could probably share code.
+ *
+ * \return true if anything changed.
+ */
+static bool ui_view2d_cur_ensure_rect_in_view(View2D *v2d, const rctf *rect)
+{
+  const float rect_width = BLI_rctf_size_x(rect);
+  const float rect_height = BLI_rctf_size_y(rect);
+
+  rctf *cur = &v2d->cur;
+  const float cur_width = BLI_rctf_size_x(cur);
+  const float cur_height = BLI_rctf_size_y(cur);
+
+  bool changed = false;
+
+  /* Snap to bottom edge. Also use if rect is higher than view bounds (could be a parameter). */
+  if ((cur->ymin > rect->ymin) || (rect_height > cur_height)) {
+    cur->ymin = rect->ymin;
+    cur->ymax = cur->ymin + cur_height;
+    changed = true;
+  }
+  /* Snap to upper edge. */
+  else if (cur->ymax < rect->ymax) {
+    cur->ymax = rect->ymax;
+    cur->ymin = cur->ymax - cur_height;
+    changed = true;
+  }
+  /* Snap to left edge. Also use if rect is wider than view bounds. */
+  else if ((cur->xmin > rect->xmin) || (rect_width > cur_width)) {
+    cur->xmin = rect->xmin;
+    cur->xmax = cur->xmin + cur_width;
+    changed = true;
+  }
+  /* Snap to right edge. */
+  else if (cur->xmax < rect->xmax) {
+    cur->xmax = rect->xmax;
+    cur->xmin = cur->xmax - cur_width;
+    changed = true;
+  }
+  else {
+    BLI_assert(BLI_rctf_inside_rctf(cur, rect));
+  }
+
+  return changed;
+}
+
+/**
+ * Adjust the view so the rectangle of \a but is in view, with some extra margin.
+ *
+ * It's important that this is only executed after buttons received their final #uiBut.rect. E.g.
+ * #UI_panels_end() modifies them, so if that is executed, this function must not be called before
+ * it.
+ *
+ * \param region: The region the button is placed in. Make sure this is actually the one the button
+ *                is placed in, not just the context region.
+ */
+void UI_but_ensure_in_view(const bContext *C, ARegion *region, const uiBut *but)
+{
+  View2D *v2d = &region->v2d;
+  /* Unitialized view or region that doesn't use View2D. */
+  if ((v2d->flag & V2D_IS_INIT) == 0) {
+    return;
+  }
+
+  rctf rect = ui_but_rect_to_view(but, region, v2d);
+
+  const int margin = UI_UNIT_X * 0.5f;
+  BLI_rctf_pad(&rect, margin, margin);
+
+  const bool changed = ui_view2d_cur_ensure_rect_in_view(v2d, &rect);
+  if (changed) {
+    UI_view2d_curRect_changed(C, v2d);
+    ED_region_tag_redraw_no_rebuild(region);
+  }
+}
+
 /* -------------------------------------------------------------------- */
 /** \name Button Store
  *