That's a very good question. I'm not sure how to find out as I don't see Type(Java) in either the table's Columns tab nor by expanding the table to see the columns and looking at its properties. I can't even see an indication of it in the debug window.
However, the situation is easily replicated by creating the following database on SQL Server 2008:
create database tstdb
create table foo (
, smalltxt varchar(255)
, bigtxt varchar(max)
Then simply go to the data tab for the foo table and try to insert a row. You can edit the smalltxt column inline but typing text into the bigtxt column results in nothing being entered. There is no feedback to say why.
Double-clicking the smalltxt column results in an inline cursor showing in that cell, allowing editing. Double-clicking the bigtxt column results in the cell editor being opened instead.
I attach the column details of the smalltxt and bigtxt columns for comparison. The most likely culprit to me is that the smalltxt column has a SQL_DATA_TYPE of 12 and the bigtxt column has a SQL_DATA_TYPE of -1. The COLUMN_SIZE of bigtxt is 2147483647, which might also be a problem.
I presume DBVisualizer is opening the cell editor either if the data type isn't in a list of known char types or if the column size exceeds a certain value. I think VARCHAR(MAX) fields should be an exception here as they can hold very large chunks of text but that doesn't mean they actually will do when edited.
Just discovered that there is a difference in how the MS SQL Server driver handles VARCHAR(max) (which was the driver I tried with) compared with jTDS.
The MS driver reports the column as a standard VARCHAR which is editable inline in DbVisualizer.
jTDS however treats the column as a LOB and inline editing is then disabled in DbVisualizer (by design).
To workaround this set the "USELOBS" driver property for jTDS to "false" and then re-connect.