Multiple objects can no longer be selected by shift-clicking on them in the scene view.

Repro steps:
1. Open TestScene from attached project.
2. In the scene view, click on any object to select it.
3. Holding down shift, click on other objects in the scene.
4. Notice how new objects cannot be selected.

Expected outcome: to be able to select multiple objects by clicking on them while holding shift down.

Notes:
- Regression: appears first in 5.4.0b5
- Issue seems to be around meshes, i.e. selecting a mesh and clicking on another mesh doesn't work, but selecting a mesh and then a light works (unless the light is in front of another mesh).
- Attached gif MultiSelect_New shows new behaviour, and MultiSelect_Old shows previous (working) behaviour.