Fix secondary monitor windows not handling DPI changes

[shanselman]

Bug

Secondary monitor windows created by \CreateMonitorWindow\ (used in 'show on all monitors' mode) had no \DpiChanged\ event handler. This caused two problems when a monitor's DPI changed at runtime (e.g., docking/undocking a laptop):

1. Incorrect window sizing/positioning — the window dimensions were calculated at creation time using the initial DPI and never updated, so the edge light frame would be wrong-sized and mispositioned after a DPI change.
2. Stale cursor hole-punch coordinates — the cached \ctx.DpiScaleX/\ctx.DpiScaleY\ values went stale, causing the cursor hole-punch geometry to use incorrect DPI scaling and appear at the wrong position.

Fix

Added a \DpiChanged\ event handler to each secondary window in \CreateMonitorWindow\ that:

• Updates \ctx.DpiScaleX\ and \ctx.DpiScaleY\ from the new DPI values
• Repositions and resizes the window using \screen.WorkingArea\ divided by the new DPI scale
• Calls \UpdateMonitorGeometry(ctx)\ to rebuild the frame geometry

This mirrors the pattern already used in the main window's DPI handling logic.

[Copilot]

Pull request overview

This PR fixes DPI handling for secondary monitor windows created in "show on all monitors" mode. Previously, these windows lacked a DpiChanged event handler, causing incorrect window sizing/positioning and stale cursor hole-punch coordinates when a monitor's DPI changed at runtime (e.g., during laptop docking/undocking).

Changes:

• Added DpiChanged event handler to secondary monitor windows that updates DPI scale values, repositions/resizes the window, and rebuilds frame geometry

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[Copilot]

The DpiChanged handler captures the screen parameter in a closure, which can become stale after monitor configuration changes. When monitors are hot-plugged or DPI settings change, the Screen objects from Screen.AllScreens can be outdated.

The main window's OnDpiChanged method (lines 585-619) handles this by calling UpdateCurrentMonitorIndex() to refresh the monitor list and get a fresh screen object before using screen.WorkingArea.

Consider storing the screen's index or device name instead of the Screen object itself, then retrieve the current Screen from a refreshed Screen.AllScreens within the DpiChanged handler. This ensures the WorkingArea values are current.

[Copilot]

The DpiChanged handler unconditionally updates window position and size without checking if the change is significant. This could potentially cause update loops or unnecessary geometry recalculations.

The main window's OnDpiChanged method (lines 609-616) includes a threshold check to only update if the difference is greater than 1 pixel, which helps avoid infinite update loops.

Consider adding a similar threshold check before updating the window properties and calling UpdateMonitorGeometry.

Suggested change

	ctx.DpiScaleX = dpiArgs.NewDpi.DpiScaleX;
	ctx.DpiScaleY = dpiArgs.NewDpi.DpiScaleY;
	window.Left = screen.WorkingArea.X / ctx.DpiScaleX;
	window.Top = screen.WorkingArea.Y / ctx.DpiScaleY;
	window.Width = screen.WorkingArea.Width / ctx.DpiScaleX;
	window.Height = screen.WorkingArea.Height / ctx.DpiScaleY;
	UpdateMonitorGeometry(ctx);
	double newDpiX = dpiArgs.NewDpi.DpiScaleX;
	double newDpiY = dpiArgs.NewDpi.DpiScaleY;
	double newLeft = screen.WorkingArea.X / newDpiX;
	double newTop = screen.WorkingArea.Y / newDpiY;
	double newWidth = screen.WorkingArea.Width / newDpiX;
	double newHeight = screen.WorkingArea.Height / newDpiY;
	// Only update if the resulting geometry change is significant (more than 1 pixel)
	if (Math.Abs(window.Left - newLeft) > 1.0 ||
	Math.Abs(window.Top - newTop) > 1.0 ||
	Math.Abs(window.Width - newWidth) > 1.0 ||
	Math.Abs(window.Height - newHeight) > 1.0)
	{
	ctx.DpiScaleX = newDpiX;
	ctx.DpiScaleY = newDpiY;
	window.Left = newLeft;
	window.Top = newTop;
	window.Width = newWidth;
	window.Height = newHeight;
	UpdateMonitorGeometry(ctx);
	}