Preferences: added "Check for updates at startup" setting (no actual updater code yet...).
Cosmetic changes to several action names and descriptions.

Changed, Sound Environments mode: previously selected sound environment is now automatically selected when inserting a new sound environment item. Also it's now impossible to accept the form without selecting a sound environment.
Changed, Sound Propagation mode: single-sided lines with sound zone boundary flag are now highlightable and clickable.

Fixed, Wavefront .obj Export: long texture names were handled incorrectly causing exceptions during textures export.

Fixed, Visual mode: model visibility filtering was checked incorrectly when using "Show models for current things filter only" model rendering mode.

Things Filters window: added "Mode" setting, which allows to choose whether to apply current filter in Classic modes, Visual mode, or both.

Things Filters: things filters can now be inverted (added "Invert Filter" checkbox to the Things Filters window).

Fixed, Linedefs mode, since r2346: some event lines were not redrawn when updating insert vertex preview. 

Classic modes: changed the way event lines are rendered. Should work much faster now. Event lines are now displayed while panning the view.

Fixed, DECORATE parser, DB2 bug: the parser didn't check stream length while skipping block comments, so unclosed ones caused EndOfStreamException.

Changed, Visual mode: models and lights are much less prone to disappear when near the edges of the screen.
Fixed, Classic modes: MODELDEF model scale was not taken into account when calculating model visibility, which resulted in models with increased scaled disappearing when near screen edges. 

Fixed, Visual mode, "Auto-align textures" actions: auto alignment was applied incorrectly to middle parts of doublesided lines.
Fixed, Visual mode, "Auto-align textures" actions: in some cases generated texture offsets were not rounded to map format decimal number precision.

MD3 reader: fixed a crash when trying to load a skin when skin names were used as a relative path in a model with several surfaces (and boy this description is much longer than the fix itself :) ).

Edit Selection mode, UDMF: added "High precision positioning" setting. When enabled, thing and vertex positions will be set using floating point precision. Otherwise, they will be rounded to the nearest integer.

Changed, Internal: ZDTextParser.ReadToken() now returns empty string instead of null when it's unable to read a token. This fixes many potential crashes when trying to parse malformed data (and hopefully doesn't bring new bugs...).

Fixed, Edit Selection mode: floor/ceiling textures rotation was not converted from radians to degrees when "Transform Texture Rotation" setting was disabled.

Fixed, Visual mode, since r2336: in some cases local vertical texture offset was calculated incorrectly when checking sidedef's texture alpha.

Fixed, Visual mode, UDMF: in some cases texture offsets were changed incorrectly while dragging sidedefs with negative texture scale.

Changed, Linedef Edit window, Visual mode, UDMF: when "Edit windows remember selected tabs" setting is enabled and Front or Back tab was previously selected, the tab, which corresponds to the highlighted side will be selected.

Fixed, Engine Launcher: "%AP" test parameters placeholder was replaced with "" when current map was newly created and never saved.

Fixed, Thing Categories: in some cases several subcategories with the same name were created.

Fixed, Visual mode: z position of info lines for polyobject setup things was calculated incorrectly.
Updated ZDoom_DECORATE.cfg.

Added: nested thing categories can now be defined in DECORATE using "//$Category" special comment. Syntax is the same as in SLADE 3.
Added: thing categories defined in Game Configurations can now be nested.
Changed, Thing Edit window: thing categories now use different icons.
Fixed: Thing Edit and Vertex edit windows had incorrect help links.
Fixed: Sound Propagation and Sound Environment modes had incorrect help links.
Documentation: updated "DECORATE keys" and "Things Settings" pages.

Documentation: updated "DECORATE keys" page.

Added, Edit Selection mode, UDMF: textures offset, rotation and scale an now be updated while using the mode.
Documentation: updated "DECORATE keys" page.

DECORATE: added "//$ArgNDefault" special comment. It can be used to specify the default argument value.
Visual mode, 3d floors: sloped 3d floors are now always rendered as opaque (just like in GZDoom).
Fixed a crash when creating a new unknown thing (introduced in r2324).
Documentation: updated "Action Argument Settings" and "DECORATE keys" pages.

DECORATE: added "//$ArgNEnum" special comment. It can be used to specify an enum to use with the argument.
Documentation: updated "Action Argument Settings" and "DECORATE keys" pages.

Game Configurations: added "Thing Radius" (23) and "Thing Height" (24) action argument types. When non-zero, arguments of these types will affect the size of associated thing in Classic and Visual modes.
Game Configurations: updated things in ZDoom "Bridges" category to use argument types 23 and 24.
Documentation: added "Action Argument Settings" page, updated "Basic Settings", "Map Format Settings", "Linedefs Settings" and "Things Settings" pages in Configurations -> Game Configurations group.
Documentation: updated "DECORATE keys" page.

Replaced Hourglass3D.png with the one, which matches the style of MissingTexture3D.png and UnknownTexture3D.png.

Sector Edit Form, UDMF: Sound Sequence selector is now a drop-down populated by values parsed from SNDSEQ.
Documentation: updated "(G)ZDoom text lumps support" page.
Updated udmf_zdoom.txt.

Fixed(?), PK3: SharpCompress doesn't like it when several threads try to read PK3 archive entries at the same time.
Fixed, Visual mode, 3d floors: floor and ceiling alpha calculation was broken.

Fixed several places where KeyNotFoundException can potentially occur in ThingFlagsCompare.Compare().

Updated ZDoom_ACS.cfg and ZDoom_DECORATE.cfg.