[Marc-André Moreau (mamoreau-devolutions)]

Summary

Four fixes found while building and running DT locally: tabs could not be closed with the mouse, Debug builds crashed at startup, Iseberg completion failed on an empty line, and the first Iseberg tab started two cold pwsh processes back to back.

Tab close button did nothing (regression from #56)

#56 made the tab header's drag handlers observe handled pointer events so tab dragging works on a Button. The press on the nested close X then also bubbled to the header, whose BeginTabDrag captured the pointer away from the close button, so it never raised Click. Clicking X only activated the tab.

BeginTabDrag now ignores presses that start on a nested button. Dragging from the header still works.

Debug builds crashed at startup

Debug builds attached Avalonia developer tools twice: WithDeveloperTools() in Program.BuildAvaloniaApp and AttachDeveloperTools() in App.Initialize (added in #52). The second call threw InvalidOperationException: Developer tools have already been attached. The duplicate in App.Initialize is removed. Release builds were not affected.

Iseberg completion on an empty line showed "Operation failed"

On empty or whitespace-only input, CommandCompletion.CompleteInput returns ReplacementIndex/ReplacementLength of -1. ManagedPowerShellSession passed that through, and building the editor's replacement span threw start ('-1') must be a non-negative value. A negative range is now mapped to an empty span at the cursor, in one helper shared by the normal, nested-prompt and paused-debugger completion paths.

First Iseberg tab: one pwsh instead of two

Opening the first Iseberg tab ran PowerShellInstallation.Find, which started pwsh only to read $PSHOME, its version, .NET runtime and architecture, and then the session started the bridge pwsh. The bridge bootstrap now prints that description on one stdout line before importing the module, and PowerShellSession validates it while the module loads, so it is off the critical path.

• An installation too old to load the net8.0 module still fails with the version it found, instead of waiting for the bridge pipe to time out.
• Error messages are unchanged. PowerShellInstallation moves to Iseberg.Core; PowerShellIseRuntime is removed.
• Measured from tab request to ready (Debug build, warm): 1.70 s → 1.23 s. A first launch right after a build is still dominated by app-side JIT and disk I/O.

Merged with master (#57). #57 already includes an equivalent fix for a newer Zig on PATH, so this branch takes its version of native/Zig.ps1.

Test plan

• New WindowActionRegistrationTests.ClickingTabCloseButtonClosesTheTab drives the close button with real headless pointer input. It fails without the fix ("Click raised: False") and passes with it. Also checked by real mouse clicks in the running app.
• New IsebergNativeContinuationTests.CompletionWithNothingToReplaceAnchorsEmptySpanAtCaret covers the engine and the script editor on "" and " ". It fails without the fix (-1).
• New PortableIseInstallationTests cases: real pwsh report output parses and validates; malformed reports are rejected; a fake pwsh reporting 7.3 fails in about 0.25 s with "found 7.3.0" (it times out on the old code); a pwsh that exits without reporting, and a missing executable, keep the existing messages.
• After merging master, on Windows (Release, 0 warnings): App.Tests 606/606, Settings.Tests 206/206, UI.Tests 670/670. Linux and macOS were not run locally; the fake-pwsh tests include a POSIX shell variant.

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