Skip canvas buffer realloc during splitter drag (#105)

Assigning canvas.width/height reallocates the WebGL drawing buffer; on
Windows Chrome (ANGLE -> D3D11) a fresh buffer is magenta until drawn.
During a splitter drag the holder resizes every frame while the game
step may be skipped or lag the resize, so the raw magenta buffer shows
through for the whole drag.

Guard the per-frame buffer sync in tickJS on !_uiDragging: the last-good
buffer stretches via CSS (#theCanvas is 100% of the holder) during the
drag, and the first tick after drag-end sees the size mismatch and
reallocates once, snapping crisp. Also removes per-frame realloc +
OnCanvasResized churn during a drag. Both splitter handlers set
_uiDragging, so one guard covers both.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: vchelaru

XnaFiddle.BlazorGL/wwwroot/index.html

@@ -129,10 +129,20 @@
                 // Subtract remainder to avoid drift accumulating over time
                 window._lastTickTime = now - (elapsed % window._tickInterval);
             }
-            // Sync canvas drawing buffer to its container each frame
+            // Sync canvas drawing buffer to its container each frame — but NOT while a splitter is
+            // being dragged (issue #105). Assigning canvas.width/height REALLOCATES the WebGL drawing
+            // buffer, and on Windows Chrome (ANGLE -> D3D11) a fresh buffer is filled with magenta as
+            // its uninitialized pattern. Normally invisible because the game's Draw repaints it that
+            // same frame — but a splitter drag changes the holder size every frame while the game step
+            // may be skipped (touch, _uiDragging) or lag the resize, so the raw magenta buffer shows
+            // through for the whole drag. Instead we let the last-good buffer stretch via CSS
+            // (#theCanvas is width/height:100% of the holder) during the drag — slightly soft — and
+            // reallocate once on drag-end: the first tick after _uiDragging clears sees the size
+            // mismatch below and snaps the buffer crisp. This also removes the per-frame realloc +
+            // OnCanvasResized churn during a drag. Both splitter handlers set _uiDragging.
             var canvas = document.getElementById('theCanvas');
             var holder = document.getElementById('canvasHolder');
-            if (canvas && holder) {
+            if (canvas && holder && !window._uiDragging) {
                 var w = holder.clientWidth;
                 var h = holder.clientHeight;
                 if (canvas.width !== w || canvas.height !== h) {