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"In full screen mode, I used to be able to expand and collapse panels in the blank space to the left of the panel names, but that no longer works in LrC 11.3."
I see similar behavior in both LR 11.2 and 11.3. When I start LR (in whatever screen mode it was last in in) and go to Develop, clicking the blank space works. But as I switch screen modes with Next Screen Mode (Shift + F), often clicking the blank space stops working. When that happens clicking the blank background of the main Loupe view once restores the blank-space functionality;
"hovering over sliders and using arrow keys to make adjustments doesn't always work."
I see this in both LR 11.2 and 11.3. When I switch between screen modes, sometimes hover/arrow keys stop working (hover doesn't highlight the slider). Clicking once on the background of the Loupe view (as described above) restores the functionality.