HW: stabilize original BootMii interaction

Source/Core/Core/HW/VideoInterface.cpp

@@ -320,9 +320,12 @@ void VideoInterfaceManager::RegisterMMIO(MMIO::Mapping* mmio, u32 base)
         return 1 + (vi.m_half_line_count) / 2;
       }),
       MMIO::ComplexWrite<u16>([](Core::System& system, u32, u16 val) {
-        WARN_LOG_FMT(
-            VIDEOINTERFACE,
-            "Changing vertical beam position to {:#06x} - not documented or implemented yet", val);
+        auto& vi = system.GetVideoInterface();
+        const u32 total_half_lines = vi.GetHalfLinesPerEvenField() + vi.GetHalfLinesPerOddField();
+        const u32 line = val == 0 ? 0 : static_cast<u32>(val - 1);
+        const u32 target_half_line = line * 2 + (vi.m_half_line_count & 1);
+        vi.m_half_line_count =
+            total_half_lines == 0 ? target_half_line : target_half_line % total_half_lines;
       }));
   mmio->Register(
       base | VI_HORIZONTAL_BEAM_POSITION, MMIO::ComplexRead<u16>([](Core::System& system, u32) {
@@ -334,10 +337,20 @@ void VideoInterfaceManager::RegisterMMIO(MMIO::Mapping* mmio, u32 base)
         return std::clamp<u16>(value, 1, vi.m_h_timing_0.HLW * 2);
       }),
       MMIO::ComplexWrite<u16>([](Core::System& system, u32, u16 val) {
-        WARN_LOG_FMT(
-            VIDEOINTERFACE,
-            "Changing horizontal beam position to {:#06x} - not documented or implemented yet",
-            val);
+        auto& vi = system.GetVideoInterface();
+        const u32 half_line_width = vi.m_h_timing_0.HLW;
+        if (half_line_width == 0)
+          return;
+
+        const u32 horizontal_position =
+            std::clamp<u32>(val, 1, static_cast<u32>(half_line_width) * 2);
+        const u32 target_half_line = horizontal_position > half_line_width ? 1 : 0;
+        vi.m_half_line_count = (vi.m_half_line_count & ~1u) | target_half_line;
+
+        const u64 ticks_into_line =
+            static_cast<u64>(horizontal_position - 1) * vi.GetTicksPerHalfLine() / half_line_width;
+        const u64 ticks = system.GetCoreTiming().GetTicks();
+        vi.m_ticks_last_line_start = ticks >= ticks_into_line ? ticks - ticks_into_line : 0;
       }));
 
   // The following MMIOs are interrupts related and update interrupt status
@@ -972,9 +985,10 @@ void VideoInterfaceManager::Update(u64 ticks)
   // the beginning of a new full-line, update the timer
 
   ++m_half_line_count;
-  if (m_half_line_count == GetHalfLinesPerEvenField() + GetHalfLinesPerOddField())
+  const u32 total_half_lines = GetHalfLinesPerEvenField() + GetHalfLinesPerOddField();
+  if (total_half_lines != 0 && m_half_line_count >= total_half_lines)
   {
-    m_half_line_count = 0;
+    m_half_line_count %= total_half_lines;
   }
 
   if (!(m_half_line_count & 1))