zotLook 1.2.2
The windowed contact sheet moves to Ctrl+Shift+Tab
Shift+Alt+Space was awkward to reach. Bare Shift+Tab could not have it: Zotero moves focus backwards with that key across twenty-two targets — collections pane, item list, item pane, toolbars, tab bar — and since zotLook consumes a matched keystroke, taking it would have stopped anyone tabbing out of the item list.
With Control held, Zotero's focus handler returns at once: it drops any Tab carrying a modifier other than Shift. Its own tab switching sits on Ctrl+PageUp and Ctrl+PageDown, on macOS also on Cmd+Shift+[ / ]; its tab bar is not a XUL tabbox, so the toolkit does not claim Ctrl+Tab the way a browser does; and macOS lists no system shortcut on the Tab key at all.
Your own choice is unaffected — this is the default, and Restore defaults is what writes it.
The conflict check sees more
It was built from the main window's <key> elements, which is everything the DOM can be asked for. Zotero binds a good deal in its own keydown listeners instead, so the pane reported no conflict for Shift+Tab — worse than reporting nothing, because it sounds like an answer.
Those keys are in the registry now and named, so a warning says what holds the combination.
Install
Download zotlook-1.2.2.xpi below, then in Zotero: Tools → Add-ons → gear icon → Install Add-on From File… Installed copies update themselves.