Probably fixed probable I/O race condition when loading images.
Fixed Visual mode stuttering due to floating point precision degradation when running the editor for several days without restarting (internal timer is now reset when saving the map or creating a new one).
Fixed, Nodes Viewer, cosmetic: Nodes Viewer window position was reset after pressing the "Rebuild Nodes" button.
Added Eternity Game configurations by printz.
Updated ZDoom_ACS.cfg (CheckClass).
Updated ZDoom ACC (CheckClass).

Added, Visual mode: added support for "Change Floor Brightness to this Brightness" and "Change Ceiling Brightness to this Brightness" Boom actions.
Changed, Sectors mode: "Flip Linedefs" and "Align Linedefs" actions will no longer flip single-sided linedefs with only front side.
Fixed, Script Editor: fixed a crash when trying to update script navigator combo box when switching to a never saved ACS script.

Added "flagsrename" Game configurations property. It allows to override Thing flag names for specific things.
Added, Automap mode: added "Show locks" button (available in Hexen and UDMF map formats only).
Updated documentation.

Changed, Game Configurations: changed "basegame" value type from int to string.
Updated documentation.

Added: preview sprites are now generated from voxels. Those are used as previews and when model rendering is disabled.
Fixed: voxel support logic was outdated, which in some cases resulted in voxels not being loaded and in some weird behaviour when trying to change pitch/roll of associated things.
Updated sprites lookup logic. Now it correctly handles sprites named like NNNNA0B0.
Fixed: in some cases DECORATE parser was unable to correctly detect sprite name & frame(s) block.
Updated ZDoom_ACS.cfg.
Updated ZDoom_DECORATE.cfg.

Added "Split Joined Sectors" Edit menu item and toolbar button. When enabled, joined sectors adjacent to drawn lines will be split.
Added "doomthingrotationangles" Game Configuration property. When enabled, editor actions related to changing thing angle will snap the resulting angle to 45 degree increments. This property is set to true for vanilla game configurations.
Fixed a crash when changing game configuration from one without Thing actions support to one with them while in Things mode.
Fixed, cosmetic, DB2 bug: current editing mode button was deselected after reloading resources.
Updated documentation ("Game Configuration - Basic Settings" page).

Added, Visual mode: added "Toggle Alpha-based Texture Highlighting" action. When enabled, 3d floor textures and middle textures on double-sided lines will be highlighted only when the crosshair points at a non-transparent pixel.
Changed: multi-part sectors are now better preserved after drawing new lines.
Changed, TEXTURES parser: patch loading errors are now ignored for textures with "NullTexture" option.
Changed, Game configurations: changed default door action arguments to better match vanilla ones.
Fixed, Actions: in some cases keyboard shortcuts involving Ctrl/Alt/Shift modifier keys were displayed incorrectly.
Fixed(?): probably fixed inability to save map wad in Slade 3.
Updated ZDoom_ACS.cfg.
Updated ZDoom_DECORATE.cfg.
Updated ZDoom ACC.

Added SNDINFO parser. Currently only the "$ambient" command is parsed. The values are used to update the titles of "Ambient Sound NN" things and to populate "ambient_sounds" Game Configuration enum.
Fixed, DECORATE parser: actors, which used "replaces" keyword and didn't have DoomEdNum were ignored.
Updated documentation ("(G)ZDoom text lumps support" page).

Added "safeboundary" game configuration property. The value determines the maximum map bounding box size considered to be safe by "Check map size" error check.
Changed, Map Analysis mode: some checks are now available only under certain conditions (for example, "Check polyobjects" is now available only when the map is in Hexen or UDMF map format).
Updated documentation ("Game Configuration - Basic Settings" page).

Added, Sound Environments mode: sound environments of the same type are now colored using the same color.
Added, Sound Environments mode: current sound environment is now highlighted (can be toggled using "Toggle Highlight" action).
Added: Tag selectors now have up/down buttons.
Fixed, Sound Environments mode: sound environments were not updated after performing Undo/Redo actions.
Fixed, Sound Propagation mode: sound zones were not updated after performing Undo/Redo actions.
Internal: moved "Toggle Highlight" action to the core, also changed it's category to "View".
Internal: "Toggle Highlight" action state is now saved in the Program configuration.
Updated ZDoom_DECORATE.cfg (GetZAt).
Updated ZDoom_linedefs.cfg (Sector_SetPortal args).

Game configurations: added "sidedefcompressionignoresaction" property. When set to true, sidedefs will be compressed regardless of linedef action (DB2 behaviour). When set to false, sidedefs, which belong to a line with an action, will be skipped when compressing sidedefs. Default value is false.

Fixed, Visual mode: rendering effects of "Slope floor/ceiling to here" things (9500/9501) should not be applied when their arg0 is 0.

Fixed, Visual mode: "Rotate CW"/"Rotate CCW" actions should update thing-based effects when used on Things.
Added/restored, Visual mode: restored "Move Texture Left/Right/Up/Down by 8" actions, added "Move Texture Left/Right/Up/Down by Grid Size" actions (default keys are Ctrl+arrow keys).
Fixed, Game configurations: Hexen's Bell actor was missing "hangs" flag.

Added, Drag Vertices\Linedefs\Sectors\Edit Selection modes: sidedefs facing outside of current selection are now reattached/added/removed after moving the selection.
Currently this will work as intended only if the selection ends up either completely inside a single sector or completely outside of any sector. Also this logic won't be applied if the selection contains the lines with only start or end vertex selected.
Internal: some TextLabel refactoring.

Added: dynamically added side panel tabs now play notify animation when the side panel is collapsed.
Added, Game configurations, ZDoom: added Sector effect 90 (Skybox sector).

Added, Game configurations: added "Enum option + enum bits":26 action argument type. It can be used when an argument combines both exclusive and bit flag values.
Updated, Game configurations: some linedef action arguments now use type 26.
Updated documentation ("Game Configuration - Action Argument Settings").
Updated ZDoom ACC (APROP_MaxStepHeight and APROP_MaxDropOffHeight).

Added, Game configurations: added "Polyobject number":25 argument type. When set, used polyobject numbers will be displayed in the action argument dropdown.
Added, Map Analysis mode: Polyobj_ExplicitLine action was not checked by "Check polyobjects" check. 
Updated ZDoom_linedefs.cfg and Hexen_linedefs.cfg (polyobject number linedef args now have type 25).
Updated documentation ("Game Configuration - Action Argument Settings").

Fixed, Things mode: in some cases selection labels were not updated after editing a thing.
Fixed, Things mode: selection labels were positioned incorrectly on things with FixedSize setting.
Fixed, Sectors mode: fixed a crash when selecting self-referencing sector when selection labels were enabled.
Fixed, Visual mode: in some cases Auto-align texture actions were not working when "use long texture names" Map Options setting was enabled.
Fixed, MD2/MD3 loader: available animation frames upper bound check was performed incorrectly, which would cause a crash in some very special cases.
Fixed, Game configurations: most Hexen/ZDoom teleport actions use TeleportDests as teleport targets, not MapSpots.

Added, Game configurations: added "targetclasses" argument property. Can be used with argument type 14 (Thing Tag). When set, only things of given classes will be shown in the argument dropdown list in Edit Things window.
Changed, Classic modes: when "Fixed Things Scale" option is enabled, things will now switch to fixed size rendering mode when their visible size is less than 48 pixels.
Changed, Classic modes: changed the fixed size of things with FixedSize setting to 28 pixels.
Updated documentation ("targetclasses" argument property).

Fixed occasional TreeView flickering in Edit Things window, Browse Action window and Tag Explorer panel.
Updated Thing category icons in the Edit Things window. They now have "opened" and "closed" states.
Internal: added BufferedTreeView to the core controls.
Updated ZDoom game configurations (sector crush mode).
Updated ZDoom ACC.

Fixed, Game configurations: some Generic_Floor:200 and Generic_Ceiling:201 parameters were named incorrectly.

Changed, Select Similar Sectors action: when "Effect" option is enabled, all sectors with at least one matching generalized/predefined effect will be selected.
Added, Tag Explorer plugin: a separate category for each generalized/predefined effect is now created when "Sort by action special" sort mode is used.
Added, Edit Effect window: normal and generalized effects can now be set at the same time.
Fixed, Edit Action window: in some cases Generalized actions were incorrectly processed.
Fixed, Edit Effect window: in some cases Generalized effects were incorrectly processed.
Fixed, Select Similar window: Tab control was incorrectly anchored.
Fixed, Nodes Viewer mode, cosmetic: segs angles were calculated incorrectly when showing nodes in classic format.
Fixed: HiRes textures, which didn't override any texture or flat were not loaded. 
Fixed, Tag Explorer plugin: linedef action categories were missing title when "Sort by action special" sort mode was used.
Cosmetic: renamed "Grid Setup" action to "Grid and Backdrop Setup".

TEXTURES support: both patch Blend syntaxes are now supported (previously only "Blend <string color>[,<float alpha>]" was supported).
Added X11R6RGB parser.
Rewritten parts of unhandled exceptions handling to reduce the chance of additional exceptions happening after the first one.

Game configurations: added 2 secret Sector_Set3dFloor flags. Shhh! Don't tell anybody!

Fixed, Map Analysis mode: fixed a crash when running "Check Polyobjects" check on a map without lines using "Polyobj_StartLine" action.
Updated ZDoom_linedefs.cfg (Line_SetPortalTarget).
Updated ZDoom_ACS.cfg (Line_SetPortalTarget).

Changed, Game configurations: "Polyobject Anchor" things should not trigger "Thing outside the map geometry" error in the Map Analysis mode.
Updated ZDoom_DECORATE.cfg (A_SetScale).

Fixed, Game configurations: Point Pusher (5001) and Point Puller (5002) things were missing from Boom configs.

Fixed, Script Editor: fixed a crash when trying to open Script Editor in a Doom map format map.

Added, DECORATE parser: damage types are now parsed.
Added: the editor now reports duplicate textures/flats/patches/sprites/colormaps/voxels in the loaded wads.
Added, all text parsers: added #region/#endregion support.
Added TERRAIN parser.
Added, Script Editor: added special handling for DECORATE special comments.
Added, Sector Edit window, UDMF: Soundsequence value was setup incorrectly when showing the window for multiple sectors with mixed Soundsequence value. 
Fixed, Map Options window: "Strictly load patches between P_START and P_END" was not applied when applying the changes.
Fixed, MAPINFO parser: MapInfo should be treated as defined when a map MAPINFO block corresponding to current map is encountered even if it doesn't define any properties recognized by the editor.
Fixed, all text parsers: in some cases error line was calculated incorrectly when reporting an error detected by a text parser.
Cosmetic: changed ' to " in the rest of Error and Warning messages.
Internal: added text resource tracking.
Updated ZDoom_DECORATE.cfg.
Updated documentation ("Game Configuration - Basic Settings" page).

Added, Edit Things window, UDMF: added basic User Variable support. User variables are now automatically shown in the "Custom" tab, are marked with different color and are applied only to actors, which define them.
Fixed, Script Editor: auto-complete can now show the same term several times when it's case is different (like DECORATE's "Bright" keyword and "BRIGHT" flag).
Game configurations, ZDoom: updated "Scroll_Floor" and "Scroll_Ceiling" linedef action argument definitions.
Updated ZDoom_ACS.cfg (Ceiling_CrushAndRaiseSilentDist).
Updated ZDoom_DECORATE.cfg (A_SetChaseThreshold).

Script Editor, Script Editor configurations: added additional keywords category for properties/operators. Added a separate color for those as well.
Fixed, Script Editor: auto-completion and syntax highlighting now works for properties with dots (like "Powerup.Color") and colon (like "See:").
Fixed, Linedef info panel, Hexen map format, cosmetic: activation type should be enabled when the line has an action assigned, even if it's default activation.
Updated ZDoom_linedefs.cfg (Line_SetPortal).

Added, Visual mode: "Raise/Lower Floor/Ceiling to adjacent Sector" actions now take surrounding 3d floors into account.
Added, Visual mode: highlight/selection based on texture alpha now works on 3d floors.
Changed, Game configurations, TranslucentLine:208 action: renamed the second argument from "Translucency Amount" to "Opacity".
Internal, documentation: updated udmf_zdoom.txt.

Fixed, DB2 bug: hlsl effect stream was not rewound when trying to compile it in debug mode, which resulted in always receiving IndexOutOfRangeException when trying to do it.
Fixed, Visual mode: textures on floors, ceilings and single-sided wall parts are now rendered ignoring texture alpha.
Changed, Game configurations, Sector_Set3dFloor:160 action: renamed the fourth argument from "Alpha" to "Opacity".

Fixed a crash when trying to get map title after closing Map options window.
Updated game configurations (Sector_SetPortal).

Added "Toggle sky rendering" action and toolbar button.
Added, MAPINFO parser: the editor now recognizes map number from classic map definition format. Hexen MAPINFO is now properly parsed.
Added, MAPINFO support: if current map definition contains map name as a text string, it will be shown in the editor's header.
Changed: "Toggle dynamic lights rendering" now toggles between "Don't show dynamic lights" and "Show dynamic lights" when used in Classic modes. 
Changed, Things mode: lowered the opacity of dynamic light radii.
Changed, (G)ZDoom text parsers: empty include files now trigger a warning instead of an error and no longer abort parsing.
Fixed, Game configurations: moved Stalagmite:5050 thing to Doom block (so the editor no longer tries to load it for non-Doom game configurations).
Fixed(?), Visual mode: probably fixed a hard-to-trigger exception when sorting translucent geometry.
Fixed, Visual mode: floor glow effect was incorrectly applied to walls (was broken in R2452).
Internal: restructured most of MAPINFO and GLDEFS parsers. Should be more maintainable now.  
Updated ZDoom ACC.
Updated zdbsp to 1.19.
Updated documentation.

Updated ZDoom_DECORATE.cfg.

Fixed, Map Analysis mode, "Check stuck things" check: rewritten parts of the flags checking logic to allow more accurate flag checks.
Fixed, Map Analysis mode: fixed a crash when trying to dissolve an invalid sector when one of it's linedefs referenced it on the both sides.
Fixed, Sectors mode: fixed incorrect undo description when deleting sectors.
Internal: joined declaration and assignment of some more variables.

Fixed, ANIMDEFS parser: camera texture names are limited to 8 chars in ZDoom.
Fixed, Game configurations: Stalagtite:47 thing was named Stalagmite.
Changed, Game configurations: moved Stalagmite:5050 thing to the "Obstacles" category.

Fixed, Script Editor, cosmetic: we should not show error line number when we don't have one.
ZDoom acc.exe: several error types now output more detailed info.
Fixed, Visual mode: UDMF thing alpha property was applied only when UDMF renderstyle property was also set (should also be applied when renderstyle property is set in the DECORATE actor definition).
Game configurations, ZDoom family: Pistol and Gold Wand actors now use internal sprites.
Updated ZDoom ACC and ZDoom_ACS.cfg (PickActor flags).

Thing Browser Control: fixed cosmetic focusing issue. When Things list is focused, you can now start typing to change the Filter.
Edit Action window: Actions list is now automatically focused on mouse over. When Actions list is focused, you can now start typing to change the Filter.
Edit Effect window: Effects list is now automatically focused on mouse over. When Effects list is focused, you can now start typing to change the Filter.
Game Configurations: added "Gold Wand" to all ZDoom-based Heretic configurations (also a couple of cosmetic tweaks).
Game Configurations: changed "Pistol" display sprite to the one provided with (G)ZDoom.