[Fix, Test] (RenderState, DirectGLES, MG_IntegrationTest): tell a deliberately empty scissor box apart from one that was never written

MobileGL/MG_State/GLState/RenderState/RenderState.cpp

@@ -843,6 +843,21 @@ namespace MobileGL {
                     stored = box;
                     stateChanged = true;
                 }
+                // "The application has written this rectangle" is a DIFFERENT predicate from "the
+                // value moved", and the backends need the first one: glScissor(0, 0, 0, 0) as the
+                // very first scissor call leaves every stored box byte-identical to its
+                // never-written default, and that call is precisely the one whose meaning a
+                // backend must stop guessing at (see ScissorBoxWrittenMask).
+                //
+                // The transition has to count as a state change for the version too. DirectGLES'
+                // SyncRenderState early-outs on an unchanged render-state version BEFORE it
+                // reaches the span memcmp that would otherwise notice the mask, so a version-less
+                // flag flip would sit in the parameter block and never be pushed. It is a
+                // once-per-index transition, so the steady state still costs nothing.
+                if (m_parameters.ScissorBoxWrittenMask != kAllViewportsMask) {
+                    m_parameters.ScissorBoxWrittenMask = kAllViewportsMask;
+                    stateChanged = true;
+                }
                 if (stateChanged) ++m_version;
             }
 
@@ -855,9 +870,15 @@ namespace MobileGL {
                     MOBILEGL_ASSERT(false, "Scissor box index out of range: %u", index);
                     return;
                 }
-                if (m_parameters.ScissorBoxes[index] == box) return;
+                // See SetScissorBox: a first write is state even when it does not move the value,
+                // so the unchanged-value early-out may only fire once this index is already
+                // marked written.
+                const Uint32 writtenBit = 1u << index;
+                const Bool alreadyWritten = (m_parameters.ScissorBoxWrittenMask & writtenBit) != 0;
+                if (alreadyWritten && m_parameters.ScissorBoxes[index] == box) return;
 
                 m_parameters.ScissorBoxes[index] = box;
+                m_parameters.ScissorBoxWrittenMask |= writtenBit;
                 ++m_version;
             }
 

MobileGL/MG_State/GLState/RenderState/RenderState.h

@@ -328,6 +328,18 @@ namespace MobileGL {
         // turns it into a real glEnable/glDisable.
         Uint32 ScissorTestEnabledMask = 0;
         Array<IntVec4, MAX_VIEWPORTS> ScissorBoxes{}; // x, y, width, height
+        // One bit per viewport, set the first time the application writes that index's scissor
+        // rectangle - glScissor broadcasts and sets all 16, glScissorIndexed/glScissorArrayv set
+        // the indices they name. It exists because the RECTANGLE cannot answer "has the
+        // application spoken?": ScissorBoxes starts all-zero (its spec initial value is the size
+        // of a window the frontend does not know yet, see the RenderState constructor), and
+        // glScissor(0, 0, 0, 0) is a legal GL state meaning "the scissor test rejects every
+        // fragment". A backend that reads an empty rectangle as the never-written sentinel
+        // therefore INVERTS that request into "accept every fragment"; DirectGLES did exactly
+        // that and KHR-GL43.viewport_array.scissor_zero_dimension caught it. Deliberately beside
+        // ScissorBoxes so it shares their tail span (after LogicOp) and DirectGLES' span memcmp
+        // picks a transition up like any other state.
+        Uint32 ScissorBoxWrittenMask = 0;
         // glEnable(GL_CLIP_DISTANCE0 + i) for i in [0, 8), one bit each. A bitmask rather than
         // eight bools because every consumer wants the set, not an individual flag, and because
         // the SYNC_CAPABILITY/SET_CAPABILITY macros key off a "<Name>Enabled" field name that