This seems like an odd behavior.
If I highlight some text in a monograph and right-click and add the text to a clippings document, I can click on a highlighter and highlight the text but I can't use the hotkey for that highlighter. The hotkey works before the right-click action, but not after. The highlighter panel click works both before and after the right-click action.
(Clicking on the tab to make the monograph panel active makes no difference.)
???
(Windows, L9.6.0024)
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UPDATE: It appears that after the right-click action, the page number box for the monograph becomes automatically activated, as my hotkey appears there, replacing the current page number.