Make button numbers consistent across platforms

There's no universal consensus on how buttons are numbered.  Left, right,
middle as 0, 1, 2 seems to be the most common convention on modern vaguely
similar libraries, so I've gone with that.

The switch to zero-based indices will obviously break all current client code.
Particularly since now is the time to finish any breaking changes before a
stable release, I think that is better than only changing the middle and right
numbers, which would likely go unnoticed.

4 files changed, 30 insertions, 8 deletions

diff --git a/include/pugl/pugl.h b/include/pugl/pugl.h
index 71578a9..4d26604 100644
--- a/include/pugl/pugl.h
+++ b/include/pugl/pugl.h
@@ -419,6 +419,21 @@ typedef struct {
 
 /**
    Button press or release event.
+
+   Button numbers start from 0, and are ordered: primary, secondary, middle.
+   So, on a typical right-handed mouse, the button numbers are:
+
+   Left: 0
+   Right: 1
+   Middle (often a wheel): 2
+
+   Higher button numbers are reported in the same order they are represented on
+   the system.  There is no universal standard here, but buttons 3 and 4 are
+   typically a pair of buttons or a rocker, which are usually bound to "back"
+   and "forward" operations.
+
+   Note that these numbers may differ from those used on the underlying
+   platform, since they are manipulated to provide a consistent portable API.
 */
 typedef struct {
   PuglEventType  type;   ///< #PUGL_BUTTON_PRESS or #PUGL_BUTTON_RELEASE
@@ -429,7 +444,7 @@ typedef struct {
   double         xRoot;  ///< Root-relative X coordinate
   double         yRoot;  ///< Root-relative Y coordinate
   PuglMods       state;  ///< Bitwise OR of #PuglMod flags
-  uint32_t       button; ///< Button number starting from 1
+  uint32_t       button; ///< Button number starting from 0
 } PuglButtonEvent;
 
 /**
diff --git a/src/mac.m b/src/mac.m
index 5a10964..84530b4 100644
--- a/src/mac.m
+++ b/src/mac.m
@@ -448,7 +448,7 @@ handleCrossing(PuglWrapperView* view, NSEvent* event, const PuglEventType type)
     rloc.x,
     [[NSScreen mainScreen] frame].size.height - rloc.y,
     getModifiers(event),
-    (uint32_t)[event buttonNumber] + 1,
+    (uint32_t)[event buttonNumber],
   };
 
   PuglEvent pressEvent;
@@ -469,7 +469,7 @@ handleCrossing(PuglWrapperView* view, NSEvent* event, const PuglEventType type)
     rloc.x,
     [[NSScreen mainScreen] frame].size.height - rloc.y,
     getModifiers(event),
-    (uint32_t)[event buttonNumber] + 1,
+    (uint32_t)[event buttonNumber],
   };
 
   PuglEvent releaseEvent;
diff --git a/src/win.c b/src/win.c
index eb1d0d1..c7a95d5 100644
--- a/src/win.c
+++ b/src/win.c
@@ -680,22 +680,22 @@ handleMessage(PuglView* view, UINT message, WPARAM wParam, LPARAM lParam)
     view->impl->mouseTracked = false;
     break;
   case WM_LBUTTONDOWN:
-    initMouseEvent(&event, view, 1, true, lParam);
+    initMouseEvent(&event, view, 0, true, lParam);
     break;
   case WM_MBUTTONDOWN:
     initMouseEvent(&event, view, 2, true, lParam);
     break;
   case WM_RBUTTONDOWN:
-    initMouseEvent(&event, view, 3, true, lParam);
+    initMouseEvent(&event, view, 1, true, lParam);
     break;
   case WM_LBUTTONUP:
-    initMouseEvent(&event, view, 1, false, lParam);
+    initMouseEvent(&event, view, 0, false, lParam);
     break;
   case WM_MBUTTONUP:
     initMouseEvent(&event, view, 2, false, lParam);
     break;
   case WM_RBUTTONUP:
-    initMouseEvent(&event, view, 3, false, lParam);
+    initMouseEvent(&event, view, 1, false, lParam);
     break;
   case WM_MOUSEWHEEL:
     initScrollEvent(&event, view, lParam);
diff --git a/src/x11.c b/src/x11.c
index 614b7ac..ed249a8 100644
--- a/src/x11.c
+++ b/src/x11.c
@@ -741,7 +741,14 @@ translateEvent(PuglView* const view, XEvent xevent)
       event.button.xRoot  = xevent.xbutton.x_root;
       event.button.yRoot  = xevent.xbutton.y_root;
       event.button.state  = translateModifiers(xevent.xbutton.state);
-      event.button.button = xevent.xbutton.button;
+      event.button.button = xevent.xbutton.button - 1;
+      if (event.button.button == 1) {
+        event.button.button = 2;
+      } else if (event.button.button == 2) {
+        event.button.button = 1;
+      } else if (event.button.button >= 7) {
+        event.button.button -= 4;
+      }
     }
     break;
   case KeyPress: