Fix DoubleCurve to no longer flatten the first key's tangent.

GameData/KSPCommunityFixes/Settings.cfg

@@ -109,6 +109,9 @@ KSP_COMMUNITY_FIXES
   // causing loss of vessel effects audio and random volume or left/right channel weirdness.
   LostSoundAfterSceneSwitch = true
 
+  // Fix DoubleCurve flattening the tangents of the first keyframe regardless of whether tangents are supplied.
+  DoubleCurvePreserveTangents = true
+
   // Fix recovery of EVAing kerbals either causing their inventory to be recovered twice or the
   // science data they carry not being recovered, depending on the EVA kerbal variant/suit.
   EVAKerbalRecovery = true

KSPCommunityFixes/BugFixes/DoubleCurvePreserveTangents.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Collections.Generic;
+using HarmonyLib;
+using System.Reflection.Emit;
+
+namespace KSPCommunityFixes.BugFixes
+{
+    class DoubleCurvePreserveTangents : BasePatch
+    {
+        protected override Version VersionMin => new Version(1, 8, 0);
+
+        protected override void ApplyPatches(List<PatchInfo> patches)
+        {
+            patches.Add(new PatchInfo(
+                PatchMethodType.Transpiler,
+                AccessTools.Method(typeof(DoubleCurve), nameof(DoubleCurve.RecomputeTangents)),
+                this));
+        }
+
+        static IEnumerable<CodeInstruction> DoubleCurve_RecomputeTangents_Transpiler(IEnumerable<CodeInstruction> instructions)
+        {
+            // The existing function has a test if ( count == 1 ) and, if true, it
+            // will flatten the tangents of the key regardless of if it is
+            // set to autotangent or not. Since the tangents of a single-key
+            // curve don't matter, let's just make the test always false,
+            // by making it if ( count == -1 ).
+            List<CodeInstruction> code = new List<CodeInstruction>(instructions);
+            for (int i = 1; i < code.Count; ++i)
+            {
+                if (code[i].opcode == OpCodes.Ldc_I4_1 && code[i-1].opcode == OpCodes.Ldloc_1)
+                {
+                    code[i].opcode = OpCodes.Ldc_I4_M1;
+                    break;
+                }
+            }
+
+            return code;
+        }
+    }
+}

KSPCommunityFixes/KSPCommunityFixes.csproj

@@ -93,6 +93,7 @@
     <Compile Include="BasePatch.cs" />
     <Compile Include="BugFixes\AsteroidInfiniteMining.cs" />
     <Compile Include="BugFixes\EnginePlateAirstreamShieldedTopPart.cs" />
+    <Compile Include="BugFixes\DoubleCurvePreserveTangents.cs" />
     <Compile Include="Performance\CommNetThrottling.cs" />
     <Compile Include="Performance\DisableMapUpdateInFlight.cs" />
     <Compile Include="Performance\MemoryLeaks.cs" />