Note: all the items mentioned below have been backported to LTR so they will also appear in QGIS 3.4.5.
In QGIS 2.18 you always had to select a feature first before editing it. In QGIS 3 there was no such thing - editing would be done on any feature any time. Now we are making the selection of feature ("lock") available again - using right click.
In earlier QGIS 3 versions, right click on a feature would bring up a small popup menu that offers display of vertex editor. Now, right click on a feature will immediately show the vertex editor and disable editing of any other features.
When a feature is "locked", it can be again unlocked by clicking on it or by clicking on an empty area - this will bring us back to the default mode when multiple features can be edited.
When there are multiple features in one mouse location, repeated right clicks will loop over them - with every right click you will lock to a different feature. Finally when all features have been offered, no feature will be locked and further right clicks will start the loop again.
The locking is possible when clicking a feature's vertex, edge or interior (for polygons).
It is now possible to use Shift + double click to add a new vertex without placing it to a new location. Sometimes this is useful when users want to add extra vertices on the existing segments.
From the two modes of vertex tool, "all layers" used to be the default one. But from the user feedback that seemed like less common case and users prefer to have "current layer" as the default.
This feature was funded by German QGIS user group
This feature was developed by Martin Dobias (Lutra Consulting)