From 3da581233a64f1ed9216c417db975a96fa7e76cb Mon Sep 17 00:00:00 2001
From: Clyde Stubbs <2366188+clydebarrow@users.noreply.github.com>
Date: Tue, 20 Oct 2026 16:08:03 +1100
Subject: [PATCH] [st7305] Fix position of partial updates on landscape panels
 (#21172)

---
 esphome/components/st7305/st7305.cpp           |  9 +++++++--
 .../components/st7305/display/test_st7305.cpp  | 18 ++++++++++++++++--
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/esphome/components/st7305/st7305.cpp b/esphome/components/st7305/st7305.cpp
index 36b58fdb240..2b324666fb4 100644
--- a/esphome/components/st7305/st7305.cpp
+++ b/esphome/components/st7305/st7305.cpp
@@ -116,6 +116,11 @@ ST7305::Window ST7305::compute_window_() const {
     window.block_start = static_cast<uint16_t>((inv_min >> 2) / 3 * 3);
     window.block_end =
         static_cast<uint16_t>(std::min(((inv_max >> 2) / 3) * 3 + 2, static_cast<int>(window.stride) - 1));
+    // The panel mirrors the column address window (MADCTL MX) but still fills it in ascending
+    // order, so the bands are addressed from the far end of the column range.
+    const int last_band = window.stride / 3 - 1;
+    window.caset_start = static_cast<uint8_t>(this->column_start_ + last_band - window.block_end / 3);
+    window.caset_end = static_cast<uint8_t>(this->column_start_ + last_band - window.block_start / 3);
   } else {
     // RASET rows are y pairs; CASET columns are 12-pixel bands of x blocks.
     window.stride = static_cast<uint16_t>(this->width_ / 4);
@@ -125,9 +130,9 @@ ST7305::Window ST7305::compute_window_() const {
     const int block_max = this->x_high_ >> 2;
     window.block_start = static_cast<uint16_t>(block_min / 3 * 3);
     window.block_end = static_cast<uint16_t>(std::min((block_max / 3) * 3 + 2, static_cast<int>(window.stride) - 1));
+    window.caset_start = static_cast<uint8_t>(this->column_start_ + window.block_start / 3);
+    window.caset_end = static_cast<uint8_t>(this->column_start_ + window.block_end / 3);
   }
-  window.caset_start = static_cast<uint8_t>(this->column_start_ + window.block_start / 3);
-  window.caset_end = static_cast<uint8_t>(this->column_start_ + window.block_end / 3);
   return window;
 }
 
diff --git a/tests/components/st7305/display/test_st7305.cpp b/tests/components/st7305/display/test_st7305.cpp
index e447e09e14d..b357907be76 100644
--- a/tests/components/st7305/display/test_st7305.cpp
+++ b/tests/components/st7305/display/test_st7305.cpp
@@ -108,10 +108,24 @@ TEST_F(ST7305Test, WindowAlignsToAddressBlocks) {
   EXPECT_EQ(window.outer_end, 0);
   EXPECT_EQ(window.block_start, 0);
   EXPECT_EQ(window.block_end, 2);
-  EXPECT_EQ(window.caset_start, 0x12);
-  EXPECT_EQ(window.caset_end, 0x12);
+  EXPECT_EQ(window.caset_start, 0x2A);
+  EXPECT_EQ(window.caset_end, 0x2A);
   EXPECT_EQ(window.data_length(), 3u);
 
+  // A window spanning several bands is addressed from the far end of the column range.
+  this->reset_screen_();
+  this->display_.draw_pixel_at(100, 85, Color::WHITE);
+  this->display_.draw_pixel_at(368, 293, Color::WHITE);
+
+  const auto bands = this->display_.compute_window();
+  EXPECT_EQ(bands.outer_start, 50);
+  EXPECT_EQ(bands.outer_end, 184);
+  EXPECT_EQ(bands.block_start, 0);
+  EXPECT_EQ(bands.block_end, 53);
+  EXPECT_EQ(bands.caset_start, 0x19);
+  EXPECT_EQ(bands.caset_end, 0x2A);
+  EXPECT_EQ(bands.data_length(), 135u * 54u);
+
   this->reset_screen_();
   this->display_.draw_pixel_at(200, 150, Color::WHITE);