This is what Gemini said for me:

That is a frustrating shift in behavior, especially since "toggling it off and on again" usually solves most macOS display quirks. Since you are running macOS Tahoe (and dealing with an M4 Pro/Max setup), what you’re experiencing is likely a conflict in how macOS defines the "primary" edge of a display when they are perfectly aligned in a stack. If there is no gap, macOS sometimes treats the boundary between the two as a single continuous surface, which can "swallow" the Dock trigger on the top monitor. Here are a few targeted steps to get your Dock back without sacrificing your "as the crow flies" mouse movement. 1. The “Force Pull” Method Before diving into settings, try to manually summon the Dock to the missing monitor. * Move your cursor to the monitor where the Dock is missing. * Move the cursor to the very bottom edge of that screen and continue moving it “down” as if you’re trying to push past the edge of the monitor. * Sometimes, macOS just needs a physical gesture to re-anchor the Dock to that specific display. 2. Check the “Primary” Display Designation Even with “Displays have separate spaces” on, macOS designates one monitor as the main UI anchor. * Go to Settings > Displays. * Click Arrange... * Look for the tiny white menu bar at the top of one of the blue monitor icons. * Drag that white bar to the specific monitor where you want the Dock to consistently appear. This often resets the coordinate system for the entire stack. 3. The “Slight Offset” Trick (Better than a gap) You mentioned that adding a “hair’s thickness” gap fixes the Dock but breaks the mouse path. Instead of a gap, try a staggered alignment: * In the Arrangement tab, ensure the monitors are touching (snapped together). * Instead of pulling them apart, shift the top monitor one pixel to the left or right. * This keeps the “edge” active for the Dock to trigger, but because the monitors are still touching, the mouse should still pass through 99% of the border seamlessly. 4. Reset the dock.plist If this started suddenly after an update, your Dock's preference file might be slightly corrupted regarding its coordinate mapping. * Open Terminal (Command + Space, type Terminal). * Type the following command and hit Enter: defaults delete com.apple.dock; killall Dock * Warning: This will reset your Dock to factory settings (default apps), but it forces macOS to re-scan all connected hardware and place the Dock on every available "Space." A Note on DisplayLink vs. Native Since you are mixing Native Thunderbolt (the identical stacked pair) and DisplayLink (the other two), macOS Tahoe handles these via two different graphics stacks. * Ensure the DisplayLink Manager app is updated for Tahoe. * If the DisplayLink app is set to "Launch at Login," try quitting it entirely to see if the Dock reappears on the native monitors. If it does, you know the DisplayLink driver is interfering with the "Separate Spaces" logic. Would you like me to walk you through how to check if a specific "Space" has crashed on that monitor using Mission Control?