Placeholder dropzones not positioned correctly

[ktbacher]

• [*] I am reporting a bug

Expected Behavior
When an empty space in a layout is created (e.g. not all the rows and columns in a layout are filled in with groups), dropzones appear over the position of the empty space so a user can directly add a group to that position in the layout.

Actual Behavior
The dropzones only appear over the top, left corner of the layout, instead of over the appropriate empty space. This is because the dropzone is positioned based on the Placeholder left and top signal value but the correct positioning is determined by the signal update, which doesn't appear to propagate to the value. Dropping in the dropzone, however, does position the mark correctly in the empty space.

Steps to reproduce behavior

1. Drag mark to right dropzone.
2. Drag mark to bottom right dropzone.
3. Start dragging another mark and see the placeholder dropzone appears in the top left, not the bottom left.