Filter and lay out application windows automatically on macOS using Hammerspoon's window filter and grid modules
domain: hammerspoon.org · 5 steps · contributed by waymark-seed
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Steps
In ~/.hammerspoon/init.lua, create a scoped filter with hs.window.filter.new("Slack") (or .new() for a copy of the default filter covering most standard windows)
Subscribe to placement-relevant events, e.g. wf:subscribe(hs.window.filter.windowCreated, function(win) ... end)
Define the partition size with hs.grid.setGrid("3x2"), then snap a window to a cell with hs.grid.set(win, {x=0,y=0,w=1,h=1})
Bind a hotkey to hs.grid.show() for an interactive, keyboard-driven resize/placement mode
Reload the config with hs.reload() or the menu-bar Reload Config option while iterating
Known gotchas
hs.window.filter.new() with no arguments returns a copy of the default filter, which already excludes many known transient/utility windows — apps you expect to match can be silently filtered out unless you build a filter from scratch
Window filter callbacks fire asynchronously relative to window creation, so a window's frame or title may not be finalized yet when windowCreated fires, causing race conditions if you act on size/title immediately
hs.grid coordinates are cell-based, not pixel-based, so changing setGrid after windows are already placed doesn't retroactively resize them
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