layout: fix crash in dwindle during config reload on monitor disconnect

[brunojuliao]

What does this PR do?

Fixes a SEGV crash (bad_variant_access in CDwindleAlgorithm::newTarget()) that occurs when a config reload is triggered while a monitor is disconnecting. This commonly happens when tools like hyprdynamicmonitors rewrite monitors.conf in response to a monitor hotplug event.

How does it do it?

Three targeted changes across two files:

WorkspaceAlgoMatcher::updateWorkspaceLayouts()

1. Skip inert workspaces: When a monitor disconnects, its workspaces become inert (m_monitor is reset to null, m_id becomes WORKSPACE_INVALID). The previous code only checked for null workspace pointers but did not check inert(), leading to null dereferences when the layout algorithm tried to access the workspace's monitor.

2. Resolve fallback layout names before comparison: algoForNameTiled() falls back to "dwindle" for unrecognized layout names (e.g., "default"), but the comparison in updateWorkspaceLayouts() used the raw config string. This meant "dwindle" == "default" always failed, causing a redundant layout switchup on every config reload. During a monitor disconnect, this unnecessary switchup iterates all tiled targets on workspaces that may have stale monitor references, triggering the crash.

CDwindleAlgorithm::addTarget()

3. Defensive null checks: Validates that both the workspace and its monitor exist before accessing their members. Also handles a null focused monitor by falling back to getFirstNode() instead of dereferencing null.

How to reproduce the crash (before this fix)

1. Configure Hyprland with an external monitor and the internal display disabled (e.g., via hyprdynamicmonitors)
2. Set general:layout to a value that falls back to dwindle (e.g., "default")
3. Disconnect the external monitor (or turn it off)
4. Hyprland crashes with signal 11 (SEGV)

Crash backtrace

Hyprland received signal 11(SEGV)
#4 | std::__throw_bad_variant_access(unsigned int)
#5 | Layout::Tiled::CDwindleAlgorithm::newTarget(...)
#6 | Layout::CAlgorithm::updateTiledAlgo(...)
#7 | Layout::Supplementary::CWorkspaceAlgoMatcher::updateWorkspaceLayouts()
#8 | CConfigManager::postConfigReload(...)
#9 | CConfigManager::reload()
#10 | CConfigWatcher::onInotifyEvent()

Log tail

drm: Connector DP-2 disconnected
drm: Disabling output DP-2
drm: No format for output
drm: Modesetting eDP-2 with 1920x1080@165.00Hz

Tests

Added two hyprtester integration tests in hyprtester/src/tests/main/monitorDisconnect.cpp:

1. testCrashOnMonitorDisconnectReload — spawns tiled windows on a secondary headless monitor, removes the monitor, then triggers a config reload. Verifies Hyprland stays responsive and windows migrate to the remaining monitor.

2. testCrashOnFallbackLayoutMonitorDisconnect — sets general:layout to an unrecognized name ("default") that falls back to dwindle, then removes a monitor and reloads. Verifies the fallback name resolution doesn't cause a crash during the layout switchup.

Checklist

• Code has been formatted with clang-format
• No test regressions (ctest passes)
• No new clang-tidy violations introduced
• Changes are minimal and focused on the fix
• Integration tests added for the fix

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