The ms-Callout-container container-406 which is the msla-token-picker-container has a fixed height (550px) and width (388px).
In the Expression section using a function with using the variables/data from the flow is a daunting task due to the fact that it is implemented as a single line input with a fixed width of 320px which makes that 'work area' unusable even for simpler expressions, but absolutely forget about a complex expression that uses any logical functions, and/or/if, etc.
For example: I have a dynamic string that basically creates a dynamic string with a date/time stamp - workflow name - Title. Of this simple example, I am able to see the function in the container as it is highlighted in red font:
In the actual node, you only see: if(...) -- not very informative.
In a code editor, this would look like something like below: with line breaks and indentation so writing/debugging the expression is easier.
I think it would be an invaluable enhancement to the user interface if:
we could drag and resize the callout container
the input field to be a multiple line text input field and not single line with a fixed width of 320px that only shows the first 42 chars therefore makes working with dynamic functions/expressions impossible within the browser.