diff --git a/examples/mouse_drag_regression.nim b/examples/mouse_drag_regression.nim
new file mode 100644
index 0000000..462e7a1
--- /dev/null
+++ b/examples/mouse_drag_regression.nim
@@ -0,0 +1,24 @@
+## Browser mouse regression fixture. Inspect window.windyMouseEvents from CDP.
+import windy, vmath
+
+{.emit: """
+#include <emscripten.h>
+EM_JS(void, recordMouse, (int kind, int x, int y, int dx, int dy, int down), {
+  (window.windyMouseEvents ||= []).push({kind, x, y, dx, dy, down});
+});
+""".}
+proc recordMouse(kind, x, y, dx, dy, down: cint) {.importc, nodecl.}
+let window = newWindow("Windy mouse regression", ivec2(800, 600))
+proc snapshot(kind: cint) =
+  let position = window.mousePos.vec2 / window.contentScale
+  let delta = window.mouseDelta.vec2 / window.contentScale
+  recordMouse(kind, position.x.cint, position.y.cint,
+    delta.x.cint, delta.y.cint, cint(window.buttonDown[MouseLeft]))
+window.onButtonPress = proc(button: Button) =
+  if button == MouseLeft: snapshot(1)
+window.onButtonRelease = proc(button: Button) =
+  if button == MouseLeft: snapshot(2)
+window.onMouseMove = proc() = snapshot(3)
+window.onFrame = proc() = discard
+while not window.closeRequested:
+  pollEvents()
diff --git a/src/windy/platforms/emscripten/platform.nim b/src/windy/platforms/emscripten/platform.nim
index abbd620..eca7460 100644
--- a/src/windy/platforms/emscripten/platform.nim
+++ b/src/windy/platforms/emscripten/platform.nim
@@ -5,7 +5,15 @@ import
 # include for EMSCRIPTEN_KEEPALIVE
 {.emit: """
 #include <emscripten.h>
+EM_JS(double, windy_canvas_left, (), {
+  return Module.canvas.getBoundingClientRect().left;
+});
+EM_JS(double, windy_canvas_top, (), {
+  return Module.canvas.getBoundingClientRect().top;
+});
 """.}
+proc windy_canvas_left(): cdouble {.importc, nodecl.}
+proc windy_canvas_top(): cdouble {.importc, nodecl.}
 
 import emdefs
 
@@ -734,10 +742,24 @@ proc mouseButtonToButton(button: cushort): Button =
   else: MouseButton4
 
 # Event handlers
+proc updateMousePosition(window: Window, mouseEvent: ptr EmscriptenMouseEvent) =
+  ## Use canvas-relative coordinates for both canvas and document events.
+  let position = ivec2(
+    int32(mouseEvent.clientX.float64 - windy_canvas_left()),
+    int32(mouseEvent.clientY.float64 - windy_canvas_top())
+  )
+  window.state.mousePrevPos = window.state.mousePos
+  if window.state.hasPrevMouse:
+    window.state.perFrame.mouseDelta += position - window.state.mousePos
+  window.state.mousePos = position
+  window.state.hasPrevMouse = true
+
 proc onMouseDown(eventType: cint, mouseEvent: ptr EmscriptenMouseEvent, userData: pointer): EM_BOOL {.cdecl.} =
   let window = cast[Window](userData)
   # Ensure canvas has focus when clicked
   make_canvas_focusable()
+  window.updateMousePosition(mouseEvent)
+  window.state.mouseInside = true
   let button = mouseButtonToButton(mouseEvent.button)
   window.handleButtonPress(button)
   return 1
@@ -745,16 +767,18 @@ proc onMouseDown(eventType: cint, mouseEvent: ptr EmscriptenMouseEvent, userData
 proc onMouseUp(eventType: cint, mouseEvent: ptr EmscriptenMouseEvent, userData: pointer): EM_BOOL {.cdecl.} =
   let window = cast[Window](userData)
   let button = mouseButtonToButton(mouseEvent.button)
+  if button notin window.state.buttonDown:
+    return 0
+  window.updateMousePosition(mouseEvent)
   window.handleButtonRelease(button)
   return 1
 
 proc onMouseMove(eventType: cint, mouseEvent: ptr EmscriptenMouseEvent, userData: pointer): EM_BOOL {.cdecl.} =
   let window = cast[Window](userData)
-  window.state.mousePrevPos = window.state.mousePos
-  # Use clientX/clientY as they are more reliably populated
-  window.state.mousePos = ivec2(mouseEvent.clientX.int32, mouseEvent.clientY.int32)
-  window.state.mouseInside = true
-  window.state.perFrame.mouseDelta += window.state.mousePos - window.state.mousePrevPos
+  if not window.state.mouseInside and
+      window.state.buttonDown * {MouseLeft, MouseMiddle, MouseRight, MouseButton4} == {}:
+    return 0
+  window.updateMousePosition(mouseEvent)
   if window.onMouseMove != nil:
     window.onMouseMove()
   return 1
@@ -762,6 +786,8 @@ proc onMouseMove(eventType: cint, mouseEvent: ptr EmscriptenMouseEvent, userData
 proc onMouseEnter(eventType: cint, mouseEvent: ptr EmscriptenMouseEvent, userData: pointer): EM_BOOL {.cdecl.} =
   let window = cast[Window](userData)
   window.state.mouseInside = true
+  window.state.hasPrevMouse = false
+  window.updateMousePosition(mouseEvent)
   return 1
 
 proc onMouseLeave(eventType: cint, mouseEvent: ptr EmscriptenMouseEvent, userData: pointer): EM_BOOL {.cdecl.} =
@@ -848,6 +874,8 @@ proc onBlur(eventType: cint, focusEvent: ptr EmscriptenFocusEvent, userData: poi
   ## Releases held input before notifying the application of focus loss.
   let window = cast[Window](userData)
   clearButtonsTemplate()
+  window.state.hasPrevMouse = false
+  window.state.perFrame.mouseDelta = ivec2(0)
   if frameDispatchActive:
     pendingFocusChange = true
     return 1
@@ -883,8 +911,8 @@ proc drainDeferredWindowEvents() =
 proc setupEventHandlers(window: Window) =
   # Mouse events
   discard emscripten_set_mousedown_callback_on_thread(window.canvas, cast[pointer](window), 1, onMouseDown, EM_CALLBACK_THREAD_CONTEXT)
-  discard emscripten_set_mouseup_callback_on_thread(window.canvas, cast[pointer](window), 1, onMouseUp, EM_CALLBACK_THREAD_CONTEXT)
-  discard emscripten_set_mousemove_callback_on_thread(window.canvas, cast[pointer](window), 1, onMouseMove, EM_CALLBACK_THREAD_CONTEXT)
+  discard emscripten_set_mouseup_callback_on_thread(EMSCRIPTEN_EVENT_TARGET_DOCUMENT, cast[pointer](window), 1, onMouseUp, EM_CALLBACK_THREAD_CONTEXT)
+  discard emscripten_set_mousemove_callback_on_thread(EMSCRIPTEN_EVENT_TARGET_DOCUMENT, cast[pointer](window), 1, onMouseMove, EM_CALLBACK_THREAD_CONTEXT)
   discard emscripten_set_mouseenter_callback_on_thread(window.canvas, cast[pointer](window), 1, onMouseEnter, EM_CALLBACK_THREAD_CONTEXT)
   discard emscripten_set_mouseleave_callback_on_thread(window.canvas, cast[pointer](window), 1, onMouseLeave, EM_CALLBACK_THREAD_CONTEXT)
 
diff --git a/tests/browser_mouse_drag.js b/tests/browser_mouse_drag.js
new file mode 100644
index 0000000..d1db580
--- /dev/null
+++ b/tests/browser_mouse_drag.js
@@ -0,0 +1,32 @@
+// Run in the mouse_drag_regression.html fixture after its WASM initializes.
+(() => {
+  const c = document.querySelector('canvas');
+  window.windyMouseEvents = [];
+  function mouse(target, type, x, y, buttons = 0) {
+    target.dispatchEvent(new MouseEvent(type, {
+      clientX: x, clientY: y, button: 0, buttons, bubbles: true
+    }));
+  }
+  mouse(c, 'mouseenter', 100, 100);
+  mouse(c, 'mousemove', 100, 100);
+  // No preceding move: activation clicks must still use their own coordinates.
+  mouse(c, 'mousedown', 220, 180, 1);
+  mouse(c, 'mouseleave', 810, 180, 1);
+  mouse(document, 'mousemove', 820, 190, 1);
+  mouse(document, 'mouseup', 830, 200);
+  mouse(c, 'mouseenter', 300, 200);
+  mouse(c, 'mousemove', 300, 200);
+  const events = window.windyMouseEvents;
+  const press = events.find(e => e.kind === 1);
+  const release = events.find(e => e.kind === 2);
+  const results = {
+    pressUsesCurrentPosition: press?.x === 220 && press?.y === 180,
+    dragOutsideDelivered: events.some(e => e.kind === 3 && e.x === 820),
+    releaseOutsideDelivered: release?.x === 830 && release?.y === 200,
+    buttonUpAfterReentry: events.at(-1)?.down === 0
+  };
+  if (!Object.values(results).every(Boolean)) {
+    throw Error(JSON.stringify({results, events}));
+  }
+  return {results, events};
+})()