The switch is great!
I tested "Find in Projects" and "Find Usages". The "Search results" pane works exactly as I would expect.
However the "Usages" pane doesn't remember the last switch state.
Also it sometimes doesn't enable the preview although the button state indicates that it is enabled:

haven't had time yet to look through everything yet, but I found a problem with the refactoring preview, see fix in:
https://github.com/mbien/netbeans/commits/preview-disable/

first commit might be squashable with the cleanup commit, second commit has the fixes, see msg. (feel free to squash)

@dschniedertuens-tc thanks for testing, indeed the implementation for the refactoring panel was off. An updated version is available here: https://github.com/apache/netbeans/suites/35751671606/artifacts/2760443851

@mbien thanks for testing. I plan to squash the cleanup commit into the first cleanup (there is a conflict, but that is trivial to resolve). The second commit more or less matched my initial thoughts after looking into the initial testing report, however I added some more fixes so went with my already started work.

I can confirm that the preview switch for the usages panel now also works as I would expect.
Thank you for your work on it!

this avoids out of bounds issues:

diff --git a/ide/diff/src/org/netbeans/modules/diff/builtin/visualizer/editable/DiffViewManager.java b/ide/diff/src/org/netbeans/modules/diff/builtin/visualizer/editable/DiffViewManager.java
index 40ecfd9..42df713 100644
--- a/ide/diff/src/org/netbeans/modules/diff/builtin/visualizer/editable/DiffViewManager.java
+++ b/ide/diff/src/org/netbeans/modules/diff/builtin/visualizer/editable/DiffViewManager.java
@@ -505,7 +505,7 @@
             if (checkFileEdge && rightOffet >= rightPane.getScrollPane().getVerticalScrollBar().getMaximum()) {
                 rightOffet = map.length - 1;
             }
-            if (rightOffet >= map.length) return;
+            if (rightOffet >= map.length || rightOffet < 0) return;
             leftPane.getScrollPane().getVerticalScrollBar().setValue(map[rightOffet]
                     - halfScreen);
         }

This happens when preview is off and the refactoring view changes selection. The diff panel tries to scroll even though it is not visible. This sounds like unnecessary work given that it is not visible but i think it is probably ok, because if you toggle it on again it will show at the right position - which is what you would expect from it I think.

java.lang.ArrayIndexOutOfBoundsException: Index -1 out of bounds for length 0
	at org.netbeans.modules.diff.builtin.visualizer.editable.DiffViewManager.smartScroll(DiffViewManager.java:509)
	at org.netbeans.modules.diff.builtin.visualizer.editable.DiffViewManager.access$300(DiffViewManager.java:47)
	at org.netbeans.modules.diff.builtin.visualizer.editable.DiffViewManager$2.run(DiffViewManager.java:154)
	at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:318)
	at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723)
	at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:702)
	at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:136)
[catch] at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
	at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
	at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
	at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
	at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

added another commit for the bookmarks preview mbien@27e652e, feel free to squash or take bits of it.

Could you take a look at #8350 Might be relevant to changes here.

@mbien thank you for the updates, which I integrated
@dschniedertuens-tc it would be great if you could give the nightly a spin: https://github.com/apache/netbeans/suites/36384122149/artifacts/2841267690
@neilcsmith-net I had a very quick look at #8350, but decided not to make this more complex as is. #8350 is a different beast, but
related.

Assuming check become green and or anybody raises a veto, I plan to merge this by end of next week.

I've been working with the linked nightly for the last few days now, constantly using search and usages and the switch still works very well for me. Now that it can be disabled, I have even found cases for which I want to enable the preview 😉
Many thanks for the great work!

Lets get this in. @mbien thanks for review and suggestions, @dschniedertuens-tc thanks for testing.