Windows: a maximized window stays clear of the taskbar (fixes #1977)

apps/vectorcraft/src/window_frame.rs

@@ -8,6 +8,13 @@
 //! same in storytold/photocraft#2401). [`service`] strips the two styles and has Windows
 //! recalculate the frame; winit puts them back whenever it changes the window flags (maximize,
 //! restore, …), so the returned guard runs every frame.
+//!
+//! Windows sizes a maximize by the frame its styles describe, so a window maximized while its
+//! caption is gone is sized without one: its rect covers the whole monitor, taskbar included, and
+//! the shell reads a caption-less window covering its monitor as a full-screen app (#1977). The
+//! guard therefore also trims a parked maximize's rect back to the work area plus its own frame,
+//! clear of the taskbar, as PhotoCraft does (storytold/photocraft#3308). winit pins the client
+//! area to the work area either way (`WM_NCCALCSIZE`), so the app's pixels do not move.
 
 /// Run once per frame; a no-op once the window is borderless.
 pub type WindowFrameFn = Box<dyn FnMut()>;
@@ -30,8 +37,15 @@ mod platform {
             _ => return None,
         };
         let hwnd = find_window(target)?;
-        strip(&hwnd);
-        Some(Box::new(move || strip(&hwnd)))
+        reapply(&hwnd);
+        Some(Box::new(move || reapply(&hwnd)))
+    }
+
+    /// One pass of the per-frame guard: the styles winit put back come off again, and a parked
+    /// maximize stays clear of the taskbar.
+    fn reapply(hwnd: &HWND) {
+        strip(hwnd);
+        keep_clear_of_the_taskbar(hwnd);
     }
 
     /// The top-level window with the given raw handle: `EnumWindows` hands out real `HWND`s, so
@@ -65,6 +79,29 @@ mod platform {
             co::SWP::FRAMECHANGED | co::SWP::NOMOVE | co::SWP::NOSIZE | co::SWP::NOZORDER | co::SWP::NOACTIVATE,
         );
     }
+
+    /// [`super::clamped_height`] for this frame's window: trim a maximized window whose rect runs
+    /// under the taskbar back to the work area plus its frame. No-op otherwise.
+    fn keep_clear_of_the_taskbar(hwnd: &HWND) {
+        if !hwnd.IsZoomed() {
+            return;
+        }
+        let (Ok(r), Ok(mi)) = (hwnd.GetWindowRect(), hwnd.MonitorFromWindow(co::MONITOR::DEFAULTTONEAREST).GetMonitorInfo()) else {
+            return;
+        };
+        let (m, w) = (mi.rcMonitor, mi.rcWork);
+        let Some(height) =
+            super::clamped_height((r.left, r.top, r.right, r.bottom), (m.left, m.top, m.right, m.bottom), (w.left, w.top, w.right, w.bottom))
+        else {
+            return;
+        };
+        let _ = hwnd.SetWindowPos(
+            HwndPlace::None,
+            POINT::new(),
+            SIZE { cx: r.right - r.left, cy: height },
+            co::SWP::NOMOVE | co::SWP::NOZORDER | co::SWP::NOACTIVATE,
+        );
+    }
 }
 
 #[cfg(not(all(windows, feature = "wgpu")))]
@@ -73,3 +110,65 @@ mod platform {
         None
     }
 }
+
+/// How far a parked maximize reaches past its monitor on the left, top and right: the resize border
+/// (about 8 pt at 100 %, more with DPI). Past that, a maximized window's rect is not parked but
+/// being dragged or snapped, and is left alone.
+const FRAME: i32 = 32;
+
+/// The height to trim a maximize to so its rect stops at the taskbar, or `None` when it already
+/// does or `rect` (`left, top, right, bottom`, desktop coordinates) is not parked at its maximize
+/// place. `monitor` is the monitor's full rect and `work` its work area (the monitor less the
+/// taskbar) in the same coordinates. The parked check is against the full monitor, because that is
+/// where Windows puts a caption-less maximize; the trim is against the work area. Platform-neutral
+/// so it is tested everywhere.
+#[cfg_attr(not(all(windows, feature = "wgpu")), allow(dead_code))]
+fn clamped_height(rect: (i32, i32, i32, i32), monitor: (i32, i32, i32, i32), work: (i32, i32, i32, i32)) -> Option<i32> {
+    let (l, t, r, b) = rect;
+    let (ml, mt, mr, _) = monitor;
+    let (_, _, _, wb) = work;
+    let side = ml - l;
+    if !(0..=FRAME).contains(&side) || !(0..=FRAME).contains(&(mt - t)) || !(0..=FRAME).contains(&(r - mr)) {
+        return None;
+    }
+    if b <= wb + side {
+        return None;
+    }
+    Some(wb + side - t)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::clamped_height;
+
+    #[test]
+    fn a_maximize_covering_the_taskbar_is_trimmed_to_the_work_area() {
+        // 1920 × 1080 with a 34 px taskbar: a caption-less maximize covers the whole monitor,
+        // taskbar included (#1977). Trimmed so the rect ends at the work area plus the frame the
+        // window was parked with.
+        let monitor = (0, 0, 1920, 1080);
+        let work = (0, 0, 1920, 1046);
+        assert_eq!(clamped_height((-7, -7, 1927, 1087), monitor, work), Some(1046 + 7 - (-7)));
+    }
+
+    #[test]
+    fn a_maximize_already_clear_of_the_taskbar_is_left_alone() {
+        // What Windows sizes a framed maximize to: the work area plus the frame.
+        assert_eq!(clamped_height((-8, -8, 1928, 1054), (0, 0, 1920, 1080), (0, 0, 1920, 1046)), None);
+    }
+
+    #[test]
+    fn a_maximize_beside_a_side_taskbar_is_left_alone() {
+        // Taskbar on the left: the work area starts further right, but the monitor's bottom is
+        // still clear, so the rect reaches nothing.
+        assert_eq!(clamped_height((-8, -8, 1928, 1088), (0, 0, 1920, 1080), (48, 0, 1920, 1080)), None);
+    }
+
+    #[test]
+    fn a_maximize_being_dragged_away_is_left_alone() {
+        let monitor = (0, 0, 1920, 1080);
+        let work = (0, 0, 1920, 1046);
+        assert_eq!(clamped_height((100, 200, 1100, 900), monitor, work), None, "moved");
+        assert_eq!(clamped_height((-500, -7, 1427, 1087), monitor, work), None, "resized");
+    }
+}