Add SignatureInformation activeParameter support.

[brownts]

No description provided.

[wyuenho]

Does lsp-mode and friends currently have the ability to highlight the active parameter?

[brownts]

Does lsp-mode and friends currently have the ability to highlight the active parameter?

Yes, it does, although simply moving the cursor over a parameter won't activate it (like what happens in Elisp), unfortunately. It seems you have to either trigger it to appear (based on the lsp-signature-auto-activate setting), or manually activate it (via M-x lsp-signature-activate), although even if you manually activate it, lsp-mode will automatically deactivate it again when you move off the line.

Oddly, Eglot behaves similar to Elisp in that you only have to move the cursor over the parameter for the signature to appear. To be honest, I wish lsp-mode behaved like that, but it doesn't seem to be setup that way.

[eval-exec]

We need this.

[wyuenho]

lsp-mode will automatically deactivate it again when you move off the line

That probably has to do with the various mouse-movement handlers lsp-mode registers.

Oddly, Eglot behaves similar to Elisp in that you only have to move the cursor over the parameter for the signature to appear.

That's because lsp-mode's eldoc-documentation-functions is only setup to use textDocument/hover, whereas eglot uses both textDocument/hover and textDocument/signatureHelp. lsp-mode's signature help is only setup to trigger after self-insert or triggered from the server.

[wyuenho]

@jcs090218 @kiennq this PR should be good to go