Round down any canvas-relative mouse event coordinates (fixes SAFE_HE…

…AP asserts) (#21429)

When using a custom target the coordinates may be doubles, which causes SAFE_HEAP to assert. These are truncated to avoid this (which is what happens anyway, only now it's explicit).

site/source/docs/api_reference/html5.h.rst

@@ -419,13 +419,13 @@ Struct
   .. c:member:: long targetX
      long targetY
 
-    These fields give the mouse coordinates mapped relative to the coordinate space of the target DOM element receiving the input events (Emscripten-specific extension).
+    These fields give the mouse coordinates mapped relative to the coordinate space of the target DOM element receiving the input events (Emscripten-specific extension; coordinates are rounded down to the nearest integer).
 
 
   .. c:member:: long canvasX
      long canvasY
 
-    These fields give the mouse coordinates mapped to the Emscripten canvas client area (Emscripten-specific extension).
+    These fields give the mouse coordinates mapped to the Emscripten canvas client area (Emscripten-specific extension; coordinates are rounded down the nearest integer).
 
 
   .. c:member:: long padding

src/library_html5.js

@@ -481,16 +481,17 @@ var LibraryHTML5 = {
 #if !DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR
     if (Module['canvas']) {
       var rect = getBoundingClientRect(Module['canvas']);
-      HEAP32[idx + {{{ C_STRUCTS.EmscriptenMouseEvent.canvasX / 4 }}}] = e.clientX - rect.left;
-      HEAP32[idx + {{{ C_STRUCTS.EmscriptenMouseEvent.canvasY / 4 }}}] = e.clientY - rect.top;
+      HEAP32[idx + {{{ C_STRUCTS.EmscriptenMouseEvent.canvasX / 4 }}}] = e.clientX - (rect.left | 0);
+      HEAP32[idx + {{{ C_STRUCTS.EmscriptenMouseEvent.canvasY / 4 }}}] = e.clientY - (rect.top  | 0);
     } else { // Canvas is not initialized, return 0.
       HEAP32[idx + {{{ C_STRUCTS.EmscriptenMouseEvent.canvasX / 4 }}}] = 0;
       HEAP32[idx + {{{ C_STRUCTS.EmscriptenMouseEvent.canvasY / 4 }}}] = 0;
     }
 #endif
+    // Note: rect contains doubles (truncated to placate SAFE_HEAP, which is the same behaviour when writing to HEAP32 anyway)
     var rect = getBoundingClientRect(target);
-    HEAP32[idx + {{{ C_STRUCTS.EmscriptenMouseEvent.targetX / 4 }}}] = e.clientX - rect.left;
-    HEAP32[idx + {{{ C_STRUCTS.EmscriptenMouseEvent.targetY / 4 }}}] = e.clientY - rect.top;
+    HEAP32[idx + {{{ C_STRUCTS.EmscriptenMouseEvent.targetX / 4 }}}] = e.clientX - (rect.left | 0);
+    HEAP32[idx + {{{ C_STRUCTS.EmscriptenMouseEvent.targetY / 4 }}}] = e.clientY - (rect.top  | 0);
 
 #if MIN_SAFARI_VERSION <= 80000 || MIN_CHROME_VERSION <= 21 // https://caniuse.com/#search=movementX
 #if MIN_CHROME_VERSION <= 76
@@ -1945,11 +1946,11 @@ var LibraryHTML5 = {
         HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.pageY / 4}}}] = t.pageY;
         HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.isChanged / 4}}}] = t.isChanged;
         HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.onTarget / 4}}}] = t.onTarget;
-        HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.targetX / 4}}}] = t.clientX - targetRect.left;
-        HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.targetY / 4}}}] = t.clientY - targetRect.top;
+        HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.targetX / 4}}}] = t.clientX - (targetRect.left | 0);
+        HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.targetY / 4}}}] = t.clientY - (targetRect.top  | 0);
 #if !DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR
-        HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.canvasX / 4}}}] = canvasRect ? t.clientX - canvasRect.left : 0;
-        HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.canvasY / 4}}}] = canvasRect ? t.clientY - canvasRect.top : 0;
+        HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.canvasX / 4}}}] = canvasRect ? t.clientX - (canvasRect.left | 0) : 0;
+        HEAP32[idx + {{{ C_STRUCTS.EmscriptenTouchPoint.canvasY / 4}}}] = canvasRect ? t.clientY - (canvasRect.top  | 0) : 0;
 #endif
 
         idx += {{{ C_STRUCTS.EmscriptenTouchPoint.__size__ / 4 }}};