first of all, if you understood there is no search box at all, than it is not accurate. There is a search box even before issuing a Ctrl+f. The point is that after a Ctrl+f, nothing gets written to the search box, even though I try to enter a search term. LO seems not responsive to any keyboard stroke unless I click the mouse.
I ran bare X, without a window manager, and without a desktop environment. The results were the same. LO is not responsive to any key stroke unless I click the mouse before any keyboard presses. More specifically, I started a bare X session, running only a terminal. As suggested, at the terminal, I issued
SAL_USE_VCLPLUGIN=gen soffice --writer
The following 2 lines were printed to the terminal:
javaldx: Could not find a Java Runtime Environment!
Warning: failed to read path from javaldx
And Writer got started. It, too, does not respond to key strokes. And there seem to be only the X mouse cursor. Not a blinking cursor showing where text will be recorded in the document. I have to click the mouse to get a blinking cursor in the document, or to have Ctrl+f accept a search string.