diff --git a/rts/Lua/LuaConstEngine.cpp b/rts/Lua/LuaConstEngine.cpp
index 579a07102f..ce9defc5e5 100644
--- a/rts/Lua/LuaConstEngine.cpp
+++ b/rts/Lua/LuaConstEngine.cpp
@@ -28,6 +28,7 @@
  * @field noHandicapForReclaim boolean Whether handicap is applied to income from reclaim
  * @field groupAddDoesntSelect boolean Whether 'group add' also selects the group (does both if false)
  * @field deadTeamsKeepUnitLimit boolean Whether engine redistributes dead team unitlimit to allies (false) or keeps it as-is (true)
+ * @field reliableLuaMapShaders boolean Whether forward-only Lua map shaders activate without a deferred draw and Spring.SetMapShader program swaps refresh cached uniform locations
  */
 
 /***
@@ -83,6 +84,7 @@ bool LuaConstEngine::PushEntries(lua_State* L)
 		LuaPushNamedBool(L, "noHandicapForReclaim", true);
 		LuaPushNamedBool(L, "groupAddDoesntSelect", true);
 		LuaPushNamedBool(L, "deadTeamsKeepUnitLimit", false);
+		LuaPushNamedBool(L, "reliableLuaMapShaders", true);
 	lua_rawset(L, -3);
 
 	lua_pushliteral(L, "textColorCodes");
diff --git a/rts/Map/SMF/SMFRenderState.cpp b/rts/Map/SMF/SMFRenderState.cpp
index b116563bd8..67ec03ff6e 100644
--- a/rts/Map/SMF/SMFRenderState.cpp
+++ b/rts/Map/SMF/SMFRenderState.cpp
@@ -94,6 +94,12 @@ void SMFRenderStateGLSL::Update(
 		for (uint32_t n = GLSL_SHADER_FWD_ADV; n <= GLSL_SHADER_DFR_ADV; n++) {
 			glslShaders[n]->LoadFromID(luaMapShaderData->shaderIDs[n - 1]);
 		}
+
+		// currShader is null from Init() (GLSL_SHADER_FWD_STD is never created for
+		// the Lua state), so re-evaluate it now that program IDs changed; otherwise
+		// HasValidShader(Normal) stays false and a forward-only Lua map shader is
+		// never selected until a deferred Lua draw happens to run first
+		SetCurrentShader(smfGroundDrawer, DrawPass::Normal);
 	} else {
 		assert(luaMapShaderData == nullptr);
 
diff --git a/rts/Rendering/Shaders/Shader.h b/rts/Rendering/Shaders/Shader.h
index 90fa492d76..8f0945ce65 100644
--- a/rts/Rendering/Shaders/Shader.h
+++ b/rts/Rendering/Shaders/Shader.h
@@ -131,6 +131,10 @@ namespace Shader {
 
 			// not needed for pre-compiled programs
 			shaderObjs.clear();
+			// cached locations/values belong to the previous program; without this,
+			// uniforms set by name (e.g. texSquare) target stale locations after a
+			// Spring.SetMapShader program swap
+			uniformStates.clear();
 		}
 
 		/// create the whole shader from a lua file