docs(user): add troubleshooting on content-sized

docs/User Guide/User Guide/Scripting/Frontend Basics/Custom Widgets/Troubleshooting.md

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+# Troubleshooting
+## Why is my widget clipped by other UI elements
+
+For performance and layout reasons, the size of widgets in Trilium is independent from its children. At CSS level, this means that the widget container has `contain: size` applied to it.
+
+This works well if the widget has a fixed size (or based on its parent container), however to make a widget resize to fit its content, apply the following change:
+
+```diff
+class MyWidget extends api.RightPanelWidget {
+
++   constructor() {
++       super();
++       this.contentSized();
++   }
+        
+}
+```
+
+Alternatively apply `contain: none` to its CSS.