Stop the Linux viewer needing libSM and libICE

.github/workflows/build-native.yml

@@ -75,6 +75,32 @@ jobs:
             exit 1
           fi
 
+      # Everything recorded as needed has to be on a machine for the library to load at all, so
+      # the list is held to what any machine that runs .NET and has a display already has. It grew
+      # once without anybody choosing it: the container's linker records every library it is
+      # handed, used or not, and raylib hands on libSM and libICE, which a stock Ubuntu lacks.
+      # The viewer could not start there, and nothing noticed until somebody ran it on one.
+      - name: Check needed libraries
+        if: runner.os == 'Linux'
+        env:
+          ALLOWED: >-
+            libc.so.6 libm.so.6 libdl.so.2 libpthread.so.0 librt.so.1 libstdc++.so.6 libgcc_s.so.1
+            libX11.so.6 libGL.so.1 ld-linux-x86-64.so.2 ld-linux-aarch64.so.1
+        run: |
+          needed=$(objdump -p build/libdiffengine_viewer.so | awk '$1 == "NEEDED" { print $2 }')
+          echo "Needed:" $needed
+          failed=0
+          for library in $needed; do
+            case " $ALLOWED " in
+              *" $library "*) ;;
+              *)
+                echo "::error::libdiffengine_viewer.so needs $library, which is not in the list of libraries it may need"
+                failed=1
+                ;;
+            esac
+          done
+          exit $failed
+
       # macOS draws with AppKit and Core Text rather than raylib and ImGui, so it is a Swift
       # package rather than a CMake project. Both --arch flags in one invocation produce a
       # universal binary, so there is no separate lipo step.

claude.md

@@ -622,6 +622,13 @@ the comment there about not caching "nothing staged" asks for.
 - Built binaries are **committed** to `src/DiffEngineViewer.{Linux,Mac}/runtimes/{rid}/native/`, so a plain
   `dotnet build` produces a shippable package and contributors never need CMake. Regenerate them
   with the `build-native` GitHub workflow, which opens a PR.
+- The Linux library is linked with `--as-needed`, and `build-native` fails a build whose `NEEDED`
+  entries are outside a short list. A library recorded there has to be installed for the viewer
+  to load at all, and the manylinux container's linker records every one it is handed: raylib
+  passes on all of FindX11's, so the binaries built there needed `libSM` and `libICE`, which a
+  stock Ubuntu lacks, where the ones built on the Ubuntu runner (whose compiler passes the flag
+  itself) had not. GLFW opens the X11 extension libraries and `libGL` itself at run time, so only
+  `libX11` is left of them.
 
 **DiffEngineTray (`src/DiffEngineTray/`):**
 - Windows Forms tray application that handles pending file diffs

native/CMakeLists.txt

@@ -109,6 +109,17 @@ target_compile_definitions(diffengine_viewer PRIVATE IMGUI_USE_WCHAR32)
 find_package(Threads REQUIRED)
 target_link_libraries(diffengine_viewer PRIVATE raylib Threads::Threads ${CMAKE_DL_LIBS})
 
+# Only the libraries something is actually called in are recorded as needed. raylib hands on
+# everything CMake's FindX11 found, which includes libSM and libICE wherever their headers are
+# installed, and nothing here or in raylib calls either. A library recorded as needed has to be
+# on the machine for this one to load at all, and a stock Ubuntu has neither of those two: the
+# viewer could not start, and every inline snapshot sent to it was staged instead of shown.
+# Ubuntu's own compiler passes this by default, which is why the binaries built on the runner
+# never had the problem and the ones built in the manylinux container did.
+if(UNIX AND NOT APPLE)
+  target_link_options(diffengine_viewer PRIVATE LINKER:--as-needed)
+endif()
+
 # The managed side probes for a bare name; keep it identical on every platform apart from the
 # platform's own prefix and extension.
 set_target_properties(diffengine_viewer PROPERTIES