The patch contains various bugfixes and game improvements such as widescreen support, and provides some GUI tweaks. It also enables the use of the FMSel fan mission loader frontend with Thief 3 and bundles the most important T3 mods. It works with the GOG and Steam versions of the game.
This issue can happen when a large font size has been selected in the Windows control panel. Fixed in 1.1.5.1.