Asset Browser: Avoid appending asset data-block when drop operator will fail

For assets, the copy callback of the drop-box would append the asset
data-block. Check if the operator's poll succeeds before calling the copy
callback.
Otherwise the data-block is "secretly" appended, which the user doesn't expect
and won't notice without checking the file data in the Outliner.

For masks I had to extend the poll function, it didn't check context
sufficiently.

1 files changed, 4 insertions, 2 deletions

diff --git a/source/blender/windowmanager/intern/wm_event_system.c b/source/blender/windowmanager/intern/wm_event_system.c
index d08051dd5fb..67372675805 100644
--- a/source/blender/windowmanager/intern/wm_event_system.c
+++ b/source/blender/windowmanager/intern/wm_event_system.c
@@ -2806,8 +2806,10 @@ static int wm_handlers_do_intern(bContext *C, wmEvent *event, ListBase *handlers
               LISTBASE_FOREACH (wmDrag *, drag, lb) {
                 const char *tooltip = NULL;
                 if (drop->poll(C, drag, event, &tooltip)) {
-                  /* Optionally copy drag information to operator properties. */
-                  if (drop->copy) {
+                  /* Optionally copy drag information to operator properties. Don't call it if the
+                   * operator fails anyway, it might do more than just set properties (e.g.
+                   * typically import an asset). */
+                  if (drop->copy && WM_operator_poll_context(C, drop->ot, drop->opcontext)) {
                     drop->copy(drag, drop);
                   }