This isn't ideal — that menu already has a large number of items and can only be accessed while a note is open. Here are two possible alternatives:
1. Show the toggle button in the main toolbar
If there plugin panels, a button is added to the main toolbar to show them in a pop-up dialog.
Advanages:
Allows opening plugin panels from any screen.
A draft implementation already exists.
This has the benefit of allowing plugins to be shown even when a note is not open. I also have a draft implementation of this on a separate branch (though it needs accessibility changes).
Disadvantages:
Adds additional icons to the toolbar, decreasing space for existing items like the notebook dropdown.
The first alternative's advantages are hard to ignore. As for the lone disadvantage, here are questions related to increasing long or heavily indented notebook (dropdown) title visibility:
Would it be possible to add a mobile tooltip (like Search has) to the notebook (dropdown) title in the toolbar? Edge case notebook names could then show inside the tooltip, wrapped to fit the viewport width.
Would it be possible to extend the notebook dropdown menu to the right viewport edge (mostly for portrait mode)? Yes, the menu edge and caret won't align. However, on-screen context and the beta feature of tapping the dropdown field to clear the dropdown should suffice.
This is off-topic, but how about adding other common actions to the toolbar when appropriate? For example, I'd find the ability to sync or check when Joplin is syncing without leaving the sidebar open helpful. I'd also find the ability to pin a few hamburger menu options to the toolbar helpful for quicker action access when viewing or editing a note.
Edit: To make the above on topic, pinning specific plugins to the toolbar could also be as helpful as pinning extensions to a desktop browser toolbar.