Add addon compatibility checklist

content/docs/reference/addon-compatibility-checklist.md

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+---
+title: Addon Compatibility Checklist
+aliases:
+  - /docs/developing/addon-compatibility-checklist
+---
+
+When developing an addon, make sure to review the categories that you're editing for any factors that need to be considered.
+
+# Non-Editor Addons
+
+## Navbar
+New addons that modify the navbar must be compatible with the following addons:
+- Customizable navigation bar
+- Expandable search bar
+
+# Editor Addons
+
+## General
+
+Any elements inside `<div id="app">` that are modified with a userscript will be wiped and restored to vanilla Scratch when the user switches to the project page. This also happens with other parent `div`s: for example, when a new button is added to the color picker, then the color picker is closed. The convention is to reinstate custom UI elements by using a `while (true)` loop with `addon.tab.waitForElement` like so:
+
+  ```js
+  while (true) {
+    let element = await addon.tab.waitForElement(/* CSS selector of desired parent element from regular Scratch */, {
+      markAsSeen: true,
+    }
+    /* Re-initialize anything related to the UI here */
+  }
+  ```
+
+## Stage Header
+New addons that add elements to the stage header must be compatible with the following addons by using their respective spaces with the [`addon.tab.appendToSharedSpace`](https://scratchaddons.com/docs/reference/addon-api/addon.tab/addon.tab.appendtosharedspace/) API:
+- 60FPS project player mode
+- Clone counter
+- Gamepad support
+- Mouse position
+- Muted project player mode
+- Pause button
+
+Other things that affect the stage header:
+- Entering and exiting full screen (the `fullscreenStageHeader` space must be used in full screen)
+- Activating Turbo Mode
+- The "reverse order of project controls" addon