Fix laggy terminal sync during sidebar drags (#1598)

* Fix sidebar drag terminal resize lag

* Add display resolution churn regression

* Prelaunch display churn helper in e2e workflow

* Use manifest handoff for display churn UI test

* Fix e2e display churn harness startup

* Resolve display churn UI test socket path

* Use marker-based socket discovery in display UI test

* Add failing sidebar drag portal regression tests

* Fix sidebar drag terminal portal resize lag

* Add failing scoped resize regression tests

* Fix terminal portal resize scheduling lag

* Add failing zsh resize prompt regression test

* Fix zsh resize prompt duplication

* Fix Sequoia sidebar resize regression

* Guard display-resolution CI runner

* Run display-resolution CI on WarpBuild

* Allow backgrounded display regression app launch

* Launch display regression app directly

* Launch display regression app via NSWorkspace

* Load display regression launch env from manifest

* Write display regression manifest in runner temp dir

* Write display regression manifest in shared tmp

* Write display regression manifest in repo scratch dir

* Launch display regression app with explicit env

* Avoid xcodebuild broken pipe in compat CI

* Launch display regression via XCUIApplication

* Harden display regression socket readiness

* Trust display socket diagnostics path

* Replace display socket probe with render diagnostics

* Write display churn start marker atomically

* Move display churn harness out of /tmp

---------

Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>

.github/workflows/test-e2e.yml

@@ -108,6 +108,7 @@ jobs:
           fi
 
       - name: Create virtual display
+        if: ${{ inputs.test_filter != 'DisplayResolutionRegressionUITests' }}
         run: |
           set -euo pipefail
           echo "=== Display before ==="
@@ -204,8 +205,64 @@ jobs:
           set -euo pipefail
           SOURCE_PACKAGES_DIR="$PWD/.ci-source-packages"
           ONLY_TESTING="-only-testing:cmuxUITests/$TEST_FILTER"
+          DISPLAY_ENV_PREFIX=()
 
-          # Start recording right before the test (after build/resolve)
+          if [ "$TEST_FILTER" = "DisplayResolutionRegressionUITests" ]; then
+            HARNESS_DIR="${RUNNER_TEMP}/cmux-display-churn-${{ github.run_id }}-${{ github.run_attempt }}"
+            mkdir -p "$HARNESS_DIR"
+            PREFIX="${HARNESS_DIR}/cmux-display-churn"
+            READY_PATH="${PREFIX}.ready"
+            DISPLAY_ID_PATH="${PREFIX}.id"
+            START_PATH="${PREFIX}.start"
+            DONE_PATH="${PREFIX}.done"
+            LOG_PATH="${PREFIX}.log"
+            MANIFEST_PATH="${HARNESS_DIR}/cmux-ui-test-display-harness.json"
+
+            rm -f "$READY_PATH" "$DISPLAY_ID_PATH" "$START_PATH" "$DONE_PATH" "$LOG_PATH" "$MANIFEST_PATH"
+
+            clang -framework Foundation -framework CoreGraphics \
+              -o /tmp/create-virtual-display scripts/create-virtual-display.m
+
+            /tmp/create-virtual-display \
+              --modes 1920x1080,1728x1117,1600x900,1440x810 \
+              --ready-path "$READY_PATH" \
+              --display-id-path "$DISPLAY_ID_PATH" \
+              --start-path "$START_PATH" \
+              --done-path "$DONE_PATH" \
+              --iterations 40 \
+              --interval-ms 40 \
+              >"$LOG_PATH" 2>&1 &
+            DISPLAY_VDISPLAY_PID=$!
+            trap 'kill "${DISPLAY_VDISPLAY_PID:-}" >/dev/null 2>&1 || true; rm -f "$MANIFEST_PATH"' EXIT
+
+            for _ in {1..120}; do
+              [ -f "$READY_PATH" ] && break
+              sleep 0.25
+            done
+            [ -f "$READY_PATH" ] || {
+              echo "Display harness failed to start" >&2
+              cat "$LOG_PATH" >&2 || true
+              exit 1
+            }
+
+            cat >"$MANIFEST_PATH" <<EOF
+          {"readyPath":"$READY_PATH","displayIDPath":"$DISPLAY_ID_PATH","startPath":"$START_PATH","donePath":"$DONE_PATH","logPath":"$LOG_PATH"}
+          EOF
+            echo "Display harness manifest: $MANIFEST_PATH"
+            cat "$MANIFEST_PATH"
+
+            DISPLAY_ENV_PREFIX=(
+              CMUX_UI_TEST_DISPLAY_READY_PATH="$READY_PATH"
+              CMUX_UI_TEST_DISPLAY_ID_PATH="$DISPLAY_ID_PATH"
+              CMUX_UI_TEST_DISPLAY_START_PATH="$START_PATH"
+              CMUX_UI_TEST_DISPLAY_DONE_PATH="$DONE_PATH"
+              CMUX_UI_TEST_DISPLAY_LOG_PATH="$LOG_PATH"
+            )
+          fi
+
+          # Start recording right before the test (after build/resolve).
+          # The display churn regression creates its own virtual display above,
+          # so recording must start after that harness is ready.
           if [ "$RECORD_VIDEO" = "true" ]; then
             DEVLIST=$( ffmpeg -f avfoundation -list_devices true -i "" 2>&1 || true )
             echo "Available devices:"
@@ -233,7 +290,7 @@ jobs:
           fi
 
           set +e
-          OUTPUT=$(xcodebuild -project GhosttyTabs.xcodeproj -scheme cmux -configuration Debug \
+          OUTPUT=$(env "${DISPLAY_ENV_PREFIX[@]}" xcodebuild -project GhosttyTabs.xcodeproj -scheme cmux -configuration Debug \
             -clonedSourcePackagesDirPath "$SOURCE_PACKAGES_DIR" \
             -disableAutomaticPackageResolution \
             -destination "platform=macOS" \