Hi. Yes, solver gets stuck sometimes (especially on large rectangles of rail tiles). The operation can be cancelled by clicking the right mouse button, perhaps it was worth to mention that function somewhere.
Yes, solver may change tiles outside of painted area, otherwise it would cause contradictions in lots of cases. In fact, it re-generates a rect enclosing all painted cells plus some gap around them, but tries to leave non-painted area as is.
I did think of adding a “line” tool, but was too lazy to write the line drawing algorithm.. but it does make sense to use it to interpolate between cursor positions in free-hand painting mode, will do it in later versions of the editor (when/if they will be made). Thanks for the suggestion.