I'm using a separate frame for lsp-ui-doc, but with this issue, I cannot focus the frame, nor scroll any of the popup, as any mouse movement would clear out the doc popup.
It is also related to #532, where any mouse movement cancels active isearch.

The installation is nothing special, here is my use-package definition

  (use-package lsp-ui
    :after lsp-mode
    :commands lsp-ui-mode
    :custom
    (lsp-ui-sideline-enable nil)
    (lsp-ui-imenu-auto-refresh 'after-save)
    (lsp-ui-doc-position 'bottom)
    (lsp-ui-doc-side 'right)
    (lsp-ui-doc-show-with-cursor nil)
    (lsp-ui-doc-show-with-mouse nil)
    (lsp-ui-doc-max-height 25)
    (lsp-ui-doc-max-width 60)
    :hook
    ((lsp-ui-imenu-mode . (lambda ()
                            (hide-mode-line-mode 1)
                            ))
     )
    )

This seems to be both caused by the same track-mouse handling, and the simplest way to fix in a buffer was to (setq-local track-mouse nil). I got a potential fix with AI generated solution, which may give us a pointer for how this can be handled correctly for various use cases. At least the fix I've got works for me, so I'm opening a PR with that.