This is a known Open GL issue, i personal have experienced this as well so heres a list of suggestions to help fix this
More information on this happening to other users
http://hammerwatch.com/forum/index.php?topic=1921.01. Update your graphics driver (The current drive your using is reverting to open gl 1.1.0 which hammerwatch editor cant use properly due to a bug on their end -_-)
2. if updating drivers the normal doesn't work you sometimes have to do a manual update forcing a later version, this tends to be the case when using Intel Video Cards. if you have other video cards (NVidea/ATI etc) then attempt to run hammerwatch with one of them otherwise you will need to update it manually, heres a site for windows that should provide some help in doing so
http://windows.microsoft.com/en-us/windows/update-driver-hardware-isnt-working-properly#update-driver-hardware-isnt-working-properly=windows-73.
LAST RESORT if your on steam, you can revert hammer watch to 1.2 which the editor tends to be more willing to cooperate, to do so you need to open Steam, right click on hammerwatch then navigate to the beta tab, once there use the code "
OldVersion120" to unlock the 1.2 version, then change it to such and redownload it.