Reference catalog of desktop GUI test strategies across Windows, macOS, and Linux. Defines the three accessibility-tree backends (Microsoft UI Automation on Windows, Apple Accessibility / XCTest on macOS, AT-SPI on Linux), the wrapper-tools that drive each backend, the cross-toolkit Electron + Qt paths, the project-marker detection table plus one-driver-per-app decision table (FlaUI / WinAppDriver / electron-playwright / QtTest / XCUITest / AT-SPI), an accessibility-first locator strategy, and a desktop test-review hazard checklist (screen-object encapsulation, locator stability, explicit waits, STA / foreground-lock / elevation). Deep operational detail (per-OS async-wait hierarchies, parallel-test policy, UAC / TCC / AT-SPI elevation hazards, the high-DPI matrix) lives in references/. Use when choosing how to test or automate a desktop GUI application on Windows, macOS, or Linux, or when reviewing an existing desktop UI test suite - the strategic reference ahead of the per-tool implementation skills.