Event System: drag events no longer default to the drag start location

This avoids transform jumping which is a problem when tweaking values a
small amount. A fix for T40549 was made box-select used the location
when the key was pressed.

While it's important for box-select or any operator where it's expected
the drag-start location is used, this is only needed in some cases.
Since the event stores the click location and the current location,
no longer overwrite the events real location. Operators that depend on
using the drag-start can use this location if they need.

In some cases the region relative cursor location (Event.mval) now needs
to be calculated based on the click location.

- Added `WM_event_drag_start_mval` for convenient access to the region
  relative drag-start location (for drag events).
- Added `WM_event_drag_start_xy` for window relative coordinates.
- Added Python property Event.mouse_prev_click_x/y

Resolves T93599.

Reviewed By: Severin

Ref D14213

1 files changed, 4 insertions, 1 deletions

diff --git a/source/blender/editors/space_node/node_relationships.cc b/source/blender/editors/space_node/node_relationships.cc
index 299e7e5658c..2eae1ce240c 100644
--- a/source/blender/editors/space_node/node_relationships.cc
+++ b/source/blender/editors/space_node/node_relationships.cc
@@ -1177,8 +1177,11 @@ static int node_link_invoke(bContext *C, wmOperator *op, const wmEvent *event)
 
   bool detach = RNA_boolean_get(op->ptr, "detach");
 
+  int mval[2];
+  WM_event_drag_start_mval(event, &region, mval);
+
   float2 cursor;
-  UI_view2d_region_to_view(&region.v2d, event->mval[0], event->mval[1], &cursor[0], &cursor[1]);
+  UI_view2d_region_to_view(&region.v2d, mval[0], mval[1], &cursor[0], &cursor[1]);
   RNA_float_set_array(op->ptr, "drag_start", cursor);
   RNA_boolean_set(op->ptr, "has_link_picked", false);