fyne-io / fyne / 39131515346

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gl: anchor the content viewport to the top of the drawable (#6599)

On X11 the framebuffer size is a live query, so a window can grow before
the resize callback has updated the canvas. The content was then painted
into a viewport smaller than the window, which OpenGL places at the
bottom-left, making the content jump vertically during a resize.

Anchor the viewport to the top of the real drawable instead, and pass the
live framebuffer size from the driver. The scissor and Capture paths are
shifted by the same offset.

The shapes' fragment shaders position fragments from gl_FragCoord, which
is relative to the drawable, so they now receive the drawable height as
their frame. Without it the rounded and shadowed shapes - the tab
indicator, button and select backgrounds, the menu shadow - were drawn at
the wrong place while the canvas caught up to the window.

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