Assuming you're using one of the word processor plugins, you can just assign a keyboard shortcut to the plugin (or use the default one for Google Docs) and press that to bring up the citation dialog, which appears on top of the word processor.

We have no current plans to support that.

We have an RTF Scan feature, and there's a third-party ODF-Scan plugin (which may not be updated for Zotero 7), but post-processing citation keys in a text document just isn't a primary way of working with Zotero. If you really want to do that, there are third-party plugins available for various text editors that will bring up the Zotero citation dialog, or you can use Quick Copy and the RTF Scan citation style or the Scannable Cite translator from the ODF-Scan plugin. But it wouldn't make sense to support a global floating window in Zotero itself when post-processing citation keys isn't a primary workflow.

In general, for just inserting a citation in any text field, you can use Quick Copy via keyboard shortcuts or drag-and-drop — e.g., Cmd-Tab to Zotero, Cmd-F, Tab to move to the items list, Cmd-Shift-C to copy a citation, Cmd-Tab back to editor, Cmd-V to paste. There are also plugins for launchers such as LaunchBar that you could use to quickly search for a given item in Zotero from anywhere and bring Zotero to the front with the item selected, at which point you could use Quick Copy and Cmd-Tab back to your app.