RFR: 8372415: Stage size should match visual window bounds

[Martin Fox]

On Mon, 24 Nov 2025 14:28:47 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

> On Windows, the `Stage.width` and `Stage.height` correspond to the window size as returned by `GetWindowRect`.
> 
> Up until Windows 10, the size of a window was identical to its visual borders. However, since Windows 10 has introduced thin visual window borders, the window manager adds an invisible border of a few pixels around the window to make it easier to resize the window. Since `GetWindowRect` returns the window size _including_ these invisible borders, the location and size of a `Stage` isn't exactly what we'd expect.
> 
> For example, if we place a `Stage` at `setX(0)` and `setY(0)`, the window appears with a small distance from the screen edge, and the window size extends a few pixels beyond its visual borders (in the following images, the screenshot size corresponds to the window size; note the invisible padding around the edges):
> <img width="300" alt="window-size-1" src="https://github.com/user-attachments/assets/76ea6861-885f-4bea-aeb7-e8e6464b7199" />
> 
> What we actually want is to have the visual borders line up with the edges of the screen, and have the window size correspond to the visual borders:
> <img width="295" alt="window-size-2" src="https://github.com/user-attachments/assets/ca6bed73-e4e7-4df6-9491-d82792bb0866" />
> 
> The implementation is quite simple: instead of `GetWindowRect`, we use `DwmGetWindowAttribute(DWMA_EXTENDED_FRAME_BOUNDS)`. This gives us the bounds of the visual window borders. If this function fails, we fall back to `GetWindowRect` (now, I don't know why `DwmGetWindowAttribute(DWMA_EXTENDED_FRAME_BOUNDS)` would ever fail... maybe an old Windows version in a remote desktop scenario?).

I was thinking that JavaFX and Windows have different ideas of where the window origin (0, 0) lies; Windows thinks it's at the top left of drop shadow area and JavaFX thinks it's at the top left of the title bar (in this PR). But you're right, what's relevant is that there are two rectangles of different sizes. I still think it could be clearer that, say, `m_insets` applies to the smaller of the two (which is the extended one).

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PR Comment: https://git.openjdk.org/jfx/pull/1982#issuecomment-3604471924