Exporting the project has issues with iframe. The purpose of the iframe was to make a simple export possible. Now that it has proven not to work, the iframe can be removed.
When hovering on a rectangle, the hovered element is often an element inside the rectangle rather than the rectangle itself. The element tree should be traversed upwards until a rectangle is found. Otherwise dragging rectangles will be finicky.
Take an existing rect and resize it. While resizing, do a right-click which opens the context menu. When the context menu is closed and all mouse buttons released, the resize operation is still on and is not possible to release.
Right click when creating new rects causes broken rects to be created.
When clicking on a canvas, often by accident, Editor Core creates a small rectangle that is very hard to remove. These can be avoided by removing all rects are under some minimum size when they are created.