Add a setting to disable diff panel floating window.PANEL_FLOATING_DISABLED=true prevents diff header detaching from the page flow and sticking to the window top border. Change-Id: Idafab7f73fb52a9165b7610ae609e0a8fe52bbd9

diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html
index a47a695..5056927 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html

@@ -141,6 +141,10 @@
       .separator {
         margin: 0 .25em;
       }
+      .noOverflow {
+        display: block;
+        overflow: auto;
+      }
       @media screen and (max-width: 50em) {
         header {
           padding: .5em var(--default-horizontal-margin);
@@ -188,6 +192,7 @@
       }
     </style>
     <gr-fixed-panel
+        floating-disabled="[[_panelFloatingDisabled]]"
         keep-on-scroll
         ready-for-measure="[[!_loading]]">
       <header>
@@ -297,26 +302,25 @@
       </div>
     </gr-fixed-panel>
     <div class="loading" hidden$="[[!_loading]]">Loading...</div>
-    <div hidden hidden$="[[_loading]]">
-      <gr-diff-preferences
-          id="diffPreferences"
-          prefs="{{_prefs}}"
-          local-prefs="{{_localPrefs}}"></gr-diff-preferences>
-      <gr-diff
-          id="diff"
-          hidden
-          hidden$="[[_loading]]"
-          is-image-diff="{{_isImageDiff}}"
-          files-weblinks="{{_filesWeblinks}}"
-          change-num="[[_changeNum]]"
-          patch-range="[[_patchRange]]"
-          path="[[_path]]"
-          prefs="[[_prefs]]"
-          project-config="[[_projectConfig]]"
-          view-mode="[[_diffMode]]"
-          on-line-selected="_onLineSelected">
-      </gr-diff>
-    </div>
+    <gr-diff
+        id="diff"
+        hidden
+        hidden$="[[_loading]]"
+        class$="[[_computeDiffClass(_panelFloatingDisabled)]]"
+        is-image-diff="{{_isImageDiff}}"
+        files-weblinks="{{_filesWeblinks}}"
+        change-num="[[_changeNum]]"
+        patch-range="[[_patchRange]]"
+        path="[[_path]]"
+        prefs="[[_prefs]]"
+        project-config="[[_projectConfig]]"
+        view-mode="[[_diffMode]]"
+        on-line-selected="_onLineSelected">
+    </gr-diff>
+    <gr-diff-preferences
+        id="diffPreferences"
+        prefs="{{_prefs}}"
+        local-prefs="{{_localPrefs}}"></gr-diff-preferences>
     <gr-rest-api-interface id="restAPI"></gr-rest-api-interface>
     <gr-storage id="storage"></gr-storage>
     <gr-diff-cursor id="cursor"></gr-diff-cursor>

diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
index 95e813a..1b918db 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js

@@ -104,6 +104,10 @@
         type: Object,
         computed: '_computeCommentSkips(_commentMap, _fileList, _path)',
       },
+      _panelFloatingDisabled: {
+        type: Boolean,
+        value: () => { return window.PANEL_FLOATING_DISABLED; },
+      },
     },
 
     behaviors: [
@@ -718,5 +722,11 @@
 
       return skips;
     },
+
+    _computeDiffClass(panelFloatingDisabled) {
+      if (panelFloatingDisabled) {
+        return 'noOverflow';
+      }
+    },
   });
 })();

diff --git a/polygerrit-ui/app/elements/shared/gr-fixed-panel/gr-fixed-panel.js b/polygerrit-ui/app/elements/shared/gr-fixed-panel/gr-fixed-panel.js
index 27cb373..fc38daa 100644
--- a/polygerrit-ui/app/elements/shared/gr-fixed-panel/gr-fixed-panel.js
+++ b/polygerrit-ui/app/elements/shared/gr-fixed-panel/gr-fixed-panel.js

@@ -18,6 +18,7 @@
     is: 'gr-fixed-panel',
 
     properties: {
+      floatingDisabled: Boolean,
       readyForMeasure: {
         type: Boolean,
         observer: '_readyForMeasureObserver',
@@ -45,6 +46,9 @@
     },
 
     attached() {
+      if (this.floatingDisabled) {
+        return;
+      }
       // Enable content measure unless blocked by param.
       if (this.readyForMeasure !== false) {
         this.readyForMeasure = true;
@@ -58,7 +62,9 @@
     detached() {
       this.unlisten(window, 'scroll', '_updateOnScroll');
       this.unlisten(window, 'resize', 'update');
-      this._observer.disconnect();
+      if (this._observer) {
+        this._observer.disconnect();
+      }
     },
 
     _readyForMeasureObserver(readyForMeasure) {
@@ -76,6 +82,9 @@
     },
 
     unfloat() {
+      if (this.floatingDisabled) {
+        return;
+      }
       this.$.header.style.top = '';
       this._headerFloating = false;
       this.customStyle['--header-height'] = '';
@@ -95,6 +104,9 @@
     },
 
     _updateDebounced() {
+      if (this.floatingDisabled) {
+        return;
+      }
       this._isMeasured = false;
       this._maybeFloatHeader();
       this._reposition();

diff --git a/polygerrit-ui/app/elements/shared/gr-fixed-panel/gr-fixed-panel_test.html b/polygerrit-ui/app/elements/shared/gr-fixed-panel/gr-fixed-panel_test.html
index 97c92b7..d813a44 100644
--- a/polygerrit-ui/app/elements/shared/gr-fixed-panel/gr-fixed-panel_test.html
+++ b/polygerrit-ui/app/elements/shared/gr-fixed-panel/gr-fixed-panel_test.html

@@ -39,8 +39,8 @@
     let sandbox;
 
     setup(() => {
-      element = fixture('basic');
       sandbox = sinon.sandbox.create();
+      element = fixture('basic');
       element.readyForMeasure = true;
     });
 
@@ -48,6 +48,14 @@
       sandbox.restore();
     });
 
+    test('can be disabled with floatingDisabled', () => {
+      element.floatingDisabled = true;
+      sandbox.stub(element, '_reposition');
+      window.dispatchEvent(new CustomEvent('resize'));
+      element.flushDebouncer('update');
+      assert.isFalse(element._reposition.called);
+    });
+
     test('header is the height of the content', () => {
       assert.equal(element.getBoundingClientRect().height, 100);
     });