Fix Diff and sub-diff views not clearing on thing deselection

When deselecting a thing, the Diff tab showed unformatted residual
content and the Attributes/Features Diff sub-tabs retained stale data.
Now destroyDiff() clears the container DOM, onThingChanged(null) resets
all state variables and destroys sub-diffs, and SubDiff.destroy() clears
its container and pending content.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: thjaeckle

ui/modules/things/subDiff.ts

@@ -111,5 +111,10 @@ export function SubDiff(targetTab, title: string) {
       diffInstance.destroy();
       diffInstance = null;
     }
+    pendingLeft = null;
+    pendingRight = null;
+    if (container) {
+      container.textContent = '';
+    }
   }
 }

ui/modules/things/thingsDiff.ts

@@ -109,9 +109,16 @@ export function ThingsDiff(targetTab) {
   const onThingChanged = (thing, isNewThingId: boolean) => {
     if (!thing) {
       currentThingId = null;
+      currentMinRevision = 1;
       currentMaxRevision = 1;
+      lastLeftJson = null;
+      lastRightJson = null;
+      viewDirty = false;
       cancelActiveProbe();
       destroyDiff();
+      for (const { subDiff } of subDiffs) {
+        subDiff.destroy();
+      }
       return;
     }
 
@@ -170,6 +177,9 @@ export function ThingsDiff(targetTab) {
       diffInstance = null;
     }
     overviewSvg = null;
+    if (dom.diffContainer) {
+      dom.diffContainer.textContent = '';
+    }
   }
 
   function onTabActivated() {