Simutrans sometimes forgets to reset the variable that stores the "target" depth between the draggings.
For example, activate Lower land (Key D), click on a tile at Z = 2, drag, you'll get a terrain at Z=1.
Then, without exiting the Lower land, click on a tile at Z = 0 and drag, you'll get a terrain at Z = -1. That's okay.
Now, without exiting the Lower land click on a tile at Z = 2 and drag, you'll get a terrain at Z = -1.
Should work correctly with r5619.
The flag that indicates 'tool is used for dragging' was not correctly reset if using the lower-tool to lower land to sea-level.
Thank you. :)