最近發生 SCSS無法自動編譯的狀況，查看 Extension and Updates，問題出在 Web Compiler被停用。
手動重新啟用後一切功能如常，原以為是一時系統秀逗，但接連發生好幾次才感覺不對勁。觀察發現，即時手動啟用，重開 Visual Studio 再開，Web Compiler 又是停用狀態。進一步檢查，則發現另一套件 Bundler & Minifier 也有相同狀況。
爬文找到一篇發生在 Web Essentials 的類似案例，似乎是擴件套件 Cache 錯亂重覆造成的。照著作者做法用 devenv /log 開啟 Visual Studio Log 模式，在 %APPDATA%\Microsoft\VisualStudio\14.0\ActivityLog.xml 記錄中找到一堆 ID 相同導致套件無法載入的錯誤訊息：
2931 ERROR Extension will not be loaded because an extension with the same ID '148ffa77-d70a-407f-892b-9ee542346862' is already loaded at C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\1KE1MKUT.YWG\...
C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\AVOVISND.T3V\ Extension Manager 2017/01/01 09:12:06.894
2932 ERROR Extension will not be loaded because an extension with the same ID '148ffa77-d70a-407f-892b-9ee542346862' is already loaded at C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\1KE1MKUT.YWG\...
C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\CY10ZHN0.GEN\ Extension Manager 2017/01/01 09:12:06.894
2933 ERROR Extension will not be loaded because an extension with the same ID 'a0ae318b-4f07-4f71-93cb-f21d3f03c6d3' is already loaded at C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\01YIXUDP.4S4\...
C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\DGLLFOVT.UKJ\ Extension Manager 2017/01/01 09:12:06.894
2934 ERROR Extension will not be loaded because an extension with the same ID '148ffa77-d70a-407f-892b-9ee542346862' is already loaded at C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\1KE1MKUT.YWG\...
C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\LW0SFC1S.34X\ Extension Manager 2017/01/01 09:12:06.894
2935 ERROR Extension will not be loaded because an extension with the same ID 'a0ae318b-4f07-4f71-93cb-f21d3f03c6d3' is already loaded at C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\01YIXUDP.4S4\...
C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\NDMEKS5M.CYT\ Extension Manager 2017/01/01 09:12:06.894
2936 ERROR Extension will not be loaded because an extension with the same ID 'a0ae318b-4f07-4f71-93cb-f21d3f03c6d3' is already loaded at C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\01YIXUDP.4S4\...
C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\R24FBB2G.B1A\ Extension Manager 2017/01/01 09:12:06.894
2937 ERROR Extension will not be loaded because an extension with the same ID 'a0ae318b-4f07-4f71-93cb-f21d3f03c6d3' is already loaded at C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\01YIXUDP.4S4\...
C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\UHHDU3FN.P1K\ Extension Manager 2017/01/01 09:12:06.894
2938 ERROR Extension will not be loaded because an extension with the same ID '148ffa77-d70a-407f-892b-9ee542346862' is already loaded at C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\1KE1MKUT.YWG\...
C:\USERS\jeffrey\APPDATA\LOCAL\MICROSOFT\VISUALSTUDIO\14.0\EXTENSIONS\WCSBI30Y.CVD\ Extension Manager
檢查各資料夾內容分別為：
*1KE1MKUT.YWG\ WebCompiler
*01YIXUDP.4S4\ Bundler and Minifier
AVOVISND.T3V\ Bundler and Minifier
CY10ZHN0.GEN\ WebCompiler
DGLLFOVT.UKJ\ Bundler and Minifier
LW0SFC1S.34X\ WebCompiler
NDMEKS5M.CYT\ Bundler and Minifier
R24FBB2G.B1A\ Bundler and Minifier
UHHDU3FN.P1K\ Bundler and Minifier
WCSBI30Y.CVD\ WebCompiler
由此推論我遇到文章所說的狀況－系統存在多個重複的 WebCompiler 與 Bundler and Minifier 資料夾，Visual Studio 啟動時檢查有誤導致擴充套件被強制停用。依照文章裡的做法，連續移除套件直到它從清單消失（在我的案例 Bundler 有六份，Web Compiler 有四份，故連移了十次才移完），再重新安裝一次，問題排除。
後來進一步發現擴件套件資料夾重複應是常見問題，已有善心人士寫好找出重複擴充套件並自動刪除的潛盾機小工具，可以瞬間排除問題，如果懶得反覆手動移除重安裝可考慮使用工具修正問題。